Trident Replacement and UK Nuclear Deterrence: Requirements in an Uncertain Future

A decision to replace Trident with a like-for-like system will see the UK remain in the nuclear-weapons business well into the second half of this century, but it is far from clear that reliance on a small, retaliatory nuclear capability for deterrence would be the best approach to an increasingly complex future nuclear-threat landscape. Andrew Futter argues that the requirements of deterrence are perhaps more blurred today than at any point in the nuclear age – a situation only likely to get worse. A more holistic and long-term view of UK nuclear policy is needed, with greater consideration given to how techno-military, strategic and, to a lesser extent, political-normative developments are likely to alter, if not transform, the nature of the future deterrence environment.

T he UK is debating the replacement of its current fleeto ff our Vanguard-class nuclear-powered submarines (SSBNs) used to carryt he Tridentnuclear weaponssystematatime of greatflux, innovation and uncertainty in the global nuclear-deterrence environment. 1 This emergings trategic contexti sb eings hapedb yarangeo f both 'supply-side'a nd 'demand-side' pressures andd ynamics stemmingf rom the developmentofnew,hi-tech military systems and capabilities, adiversification of potential nuclear threats, and an increasingly influential anti-nuclearglobal political and normativec ontext.W hile the UK nuclear debate has beenl ively and heatedsince the intention to replace the submarines wasa nnouncedi nl ate 2006, 2 thishas focused primarilyonthe current political and strategic contextand particularly on the economic and opportunity costso fr etaining nuclear forces -w ith little apparent thought aboutw hatt he strategic environment mightl ook likew hen these new submarinesa re deployeda tt he end of the 2020s. 3 Given thatt he current UK governmentw ill likely agreet om ove ahead with like-for-liker eplacementi n 2016, 4 and the fact thatthis decision will seet he UK remain parto ft he exclusive nuclear-weapons' club' well into the second halfofthis century, it is essential thatsome thoughtbegiven to whether a limited -albeit sophisticated -retaliatory nuclear weaponssystemwill continue to fulfil UK nuclear deterrence requirements in an uncertain strategic future.
Af undamentalg uiding principle of the decision to remain in the 'nuclear game' forthe next couple of generations has alwaysb een thes pectreo fa n uncertain nuclear future, but it appears to be takena sg iven thatU Kn uclear weapons-a nd deterrence through the threatofnuclear retaliation -will remain sufficientinthis unknown environment. 5 As then-PrimeM inisterT onyB lair pointedout in his foreword to the 2006 WhiteP aper on Trident renewal, 'We believe thata ni ndependentB ritish nuclear deterrenti sa ne ssential part of our insurance againstt he uncertainties and riskso ft he future.' 6 This mayw ell provet ob et he case, but ar angeo f developmentsi nr ecenty earss uggests thatt he futureo perating environment of 2030 and beyond will be farremoved from thato ft oday, and especially from thato ft he Cold War( forw hich the current systemw as designed). The key question is whethera dvancesi na nd the spreado fb allistic-missiled efences (BMD), improvements in anti-submarine warfare(ASW) capabilities, and the new challenges presentedb y' cyber'm ight mean thatU Kn uclear weaponsa re less credible, usable or efficacious in decades to come. This risk of possiblen uclear impotencei sa lso being exacerbated by currentt rends in the international normative and domestic political environmenti nw hich policy will be playedo ut -p articularly the growing trendo fa nti-nuclear public sentiment. Takentogether,the spread and increasing sophistication of these techno-military With the currentTridentsystementeringits sunsetyears, the likely natureofthe futurenuclear-threatenvironment mustbeconsidered before adecisionis made on apossible replacement. Image courtesyCrown Copyright/CPOA(Phot) TamMcDonald.
capabilities, ad iversification in the threats thatn eed to be deterred, as well as changing political contextm ight make ap olicy of 'business as usual' highly problematic-q uestioning the credibility of UK threats to use nuclear weapons, or even raising the possibility of self-deterrence. While these questions maybefor the medium and long term, it would be unwise to exclude them from the debate,t hinking and strategy that willu nderpin the decisions takena bout UK nuclear weaponsinthe nearfuture.
Accordingly,t his article proceeds in three sections:fi rst, it providess ome background and contextt oU Kn uclearweaponst hinking and strategy,b efore reviewing thec urrentd ebate about Trident replacement;s econd, it looksi n moredetail at the natureofthe supplyside military-technical pressures and demand-side strategic, normativea nd political dynamics transforming and recasting the globaln uclear-deterrence environment, ande xplainst he possible implications forthe UK; third, it examines three possiblef utured eterrence pathways and begins to outline a framework foramoreholistic and longterm viewofUKnuclear strategy.

The Natureofthe UK Nuclear-Weapons Debate
The UK joinedthe exclusivenuclearclub in October 1952.S ince then, successiveU K governmentsh aver emainedc ommitted to deploying only al imited or minimum nuclear-deterrentc apability.A ftert he WE 177f ree-fall air-delivered nuclear bomb wasr etiredf roms ervice in March 1998,the UK has relied on afleetoffour Vanguard-class submarines armed with Tridents ubmarine-launched ballistic missiles (SLBMs)t od eliver its nuclear warheads,and thus undergirdits nuclear deterrent. 7 This fleeto fVanguard-class submarines wasfi rsto rdered in 1980, became operational in the 1990sa nd is due to be retired as the service lives of the submarines finish in the 2020s. Given the substantial undertakingrequired to design, build, test and finally deploy an ew fleet of submarines by the time the current ones arer etired, the Main Gate decision on whethert op ush aheadw ith like-forliker eplacementi se xpected to be taken sometime in 2016.Afterthis, it becomes very unlikely thatt he replacement programmewill be cancelled. 8 The UK policy of minimum nuclear deterrence is based on retaining the smallest nuclear forcerequired to inflict unacceptable damageo na no pponent in order to dissuade it from anuclear or otherwise potentially existential attack on the UK, or to prevent'nuclear blackmail'. As the 2006 WhitePaper put it: 9 [O]urfocus is preventing nuclear attack. TheU K'sn uclear weaponsa re not designed form ilitary use during conflict but instead to deterand prevent nuclear blackmail and actsofaggression againsto ur vital interestst hatc annot be counteredbyother means …The UK will retain only the minimum amountof destructivep ower required to achieve our deterrence objectives.
In order to meetthis requirement, the UK operates ap olicy of continuous at-sea deterrence (CASD)w hereby one nuclear-armed Vanguard-class submarine is on patrol, hidden under the ocean'ss urface and ready to firei ts nuclear weaponsa tr elatively short notice, at all times. Given the sophistication of the nuclear propulsion systems and other stealthliket echnologies, these1 5,900-tonne, 150-metre-long boatsa re virtually undetectablew hen deployedd eep in theo cean and are( currently) viewed as being invulnerable to ap re-emptive disarming strikeo ro ther interference, thus ensuring that the UK will always be able to credibly threatenn uclear retaliation. Each Vanguard submarine is capable of carryingu pt o1 92 independently targetable 100kT-class thermonuclear warheads-t welveo n each of thes ixteen Trident SLBMsalthough the 2010 StrategicD efence and Security Review(SDSR) limited this to amaximum of eight SLBMsand forty nuclear warheadsoneach submarine. 10 The SDSR also reduced the overall nuclear-warhead stockpile to no more than 180a nd the operational stockpile from 160t o1 20. 11 This makes the UK one of the smallestn uclear-weapons powers and the onlyn uclear-armed statetorely on justone deliverysystem fori ts nuclear weapons( the Trident SLBMsonthe Vanguard submarines).
The decision to begin the processof replacing the current Vanguard fleethas driven aw ide and diverse debate,a nd following the 2010 hung parliament, led directly to the 2013 Trident Alternatives Review( TAR) mandatedb yt he Liberal Democratsaspart of their coalition deal with the ConservativeP arty.T he main objectiveo ft he TARw as to assess the viability of differento ptions open to the UK beyond like-for-liker eplacement of the Vanguard-Tridents ystem, and in particular to considerw hethert here were credible alternatives to using submarines, such as basing weapons on land or deploying them on aircraft; whether othersubmarine-based options, such as nuclearattack submarinesarmed with nuclear cruisemissiles, might work; andfinally whetheralternative, reduced or 'relaxed' nuclear postures basedo n the Vanguard-Tridents ystemw ere possible. 12 While theT AR wasn ot a direct statementofgovernmentpolicy,it concludedt hatl ike-for-liker eplacement representedt he besta nd mostc osteffectiveo ption fort he UK to meett he futuren uclear-deterrence requirements as seto ut in the 2006 WhiteP aper and 2010 SDSR, although it lefto pen the question of whethert hree rather than four boats might be required in order to achieve this,o ri ft he policy of CASD might be relaxed. 13 As ar esult,a nd given the outcome of the 2015 general election, the UK governmenti sl ikely to go ahead and begin building the next generation of nuclear-capable SSBNs in 2016.
Over the pastd ecadet he 'Trident renewal' debate has focused on costs -p articularly opportunity costsf or the UK military 14 and broader UK economy; 15 the perceived necessity of such weaponsintoday'spost-Cold War, post-9/11 security environment; the implications of Scottish independence; 16 the UK's commitment to nuclear disarmamentu nder ArticleV Io ft he Nuclear Non-Proliferation Treaty (NPT); and, morer ecently,t he morality and legality of threatening the use of nuclear weapons. 17 In this way, the debateand thereforet he decision to replace the incumbent nuclear system-i s very much ar eflectiono ft he current political climate,rather than astrategic, long-terma ssessmentn ot justo fw hat the UK requires fornuclear deterrence, buta lso whethert he existing system will remain as efficacious in al argely unknown and unpredictable future nuclearenvironment.Asthe next section explains,itisfar from guaranteed thata singlesubmarine armed with arelatively small number of nuclear warheads designed principallyf or retaliatory strikes will remain sufficientfor credible nuclear deterrence in the world of 2030, 2040 and beyond. 18

TowardsaMoreDiverse Nuclear-Deterrence Environment
While thec urrentfl eeto fVanguard submarines wasn ot deployedu ntilt he 1990s, it wasc onceived, designed and built to reflectthe exigenciesofthe Cold War. This essentially meantretaining the ability to inflict unacceptable damage on the SovietU nion (particularly Russia itself), known colloquially as the 'Moscow criterion', in an environment underpinned by the notiono fm utual assured destruction (MAD) through nuclear retaliation. However, this environmenth as begun to shift in the pastt wo decades-n ot justb ecause thes uperpowern uclear standoffh as appeared to be replacedw ith am ore complex geopolitical nuclear milieu, but also because of advancesin'supplyside' military-technological tools and capabilitiest hatm ightb eu sed for deterrence or againstn uclear forces, as well as 'demand-side' developments in global normative nuclear thinking regarding whato rw ho needs to be deterred andh ow.T he netr esult is a global deterrence environmentremoved from when the currentT ridents ystem wasfi rstc onceived and one thats eems likely to become ever-more complex as the deploymento ft he replacement submarines begins in the late 2020s.
Ac entral principleo fU Ks trategic nuclearthinking appearstobethat, in an uncertain future, anya ctor threatening the use of nuclear weaponsa gainstt he UK canb ed eterred through the threat of nuclear retaliation. As the 2006 White Paper argues: 19 It is not possible accurately to predict the global security environment over thenext20to50years. On ourcurrent analysis, we cannot rule out the risk either thatamajor direct nuclear threat to the UK'svital interestswill re-emerge or thatn ew states will emerge that possess am orel imited nuclear capability,b ut one thatc ould pose a gravethreattoour vital interests. Equally therei sar isk thats ome countries might in futureseek to sponsor nuclear terrorism from theirs oil. We must not allow such states to threateno ur national security,ortodeter us andthe internationalcommunity from taking the action required to maintain regional and global security. We canonly detersuch threats in future through the continued possession of nuclear weapons.
However, and whilet he natureo ft he futurenuclear-threatenvironment facing the UK remainsunknown (and debated), this does raise keyquestions about who the UK is trying to deterwith its nuclear forces. It is quitep ossible thatd ifferent adversaries will requiread ifferent mix of capabilities, and even 'tailored deterrence'.I ti se qually important to considerh ow this mightb ea chieved in practice, andw hethert his canr emain credibleand effectiveinthe long run.
Akey featureofthe lasttwo decades has beenan oticeable diversification in the nuclear-threatl andscape facing the UK, wherebyt he traditional focus on large strategic nuclear competitors (such as Russia and,t oal esser extent, China) has beenincreasingly augmented by as et of newn uclear actors, as part of at ransition to as o-called 'second nuclear age'. 20 In response to these new dynamics,amorediverse US (andNATO) deterrence strategy has already emerged to combatd ifferentt ypes of threats: a traditional MAD-based nuclear-centric approach to dealwith peer competitors, and am oren uanceds trategy including am ixtureo fo ffensivea nd defensive forces to dealw ith 'rogue' states and possibly non-state nuclear actorst hat mayn ot 'adhere' to MAD. 21 However, thus fart he UK has remained publicly committed to the efficacy of traditional notionsofnuclear deterrence againstall potential nuclear threats, even though it remainsu nclear thatap ostureb ased primarily on ar etaliatory nuclear force willmeetthe full spectrum of UK nucleardeterrence demands in the longer term. 22 This naturally leads to the question of whether deterrence by punishmentthatistosay,the threatofunacceptable It is quite possiblethat differentadversaries will require 'tailored deterrence' damagecaused by anuclear responsecanremain thesole basis of UK nucleardeterrence strategy,o rw hetheram ore nuancedp osturei ncluding deterrence by denial( using advancedc onventional weaponry fore xample, either instead of or as well as nuclear forces) might be required to augmentt his, and help addresst he widerg amut of potential nuclear threats (including accidents and possible unauthorised use).
The current debate is also premised, consciously or not, on the belieft hata small, retaliatory nuclear forcew ill remain highlycredible and invulnerable, and thatUKpolicy-makers as well as any potential enemyw ill be confidentt hat the threato fr etaliation cana nd will be carried out. In fact, ad irect knockon effect of thec hanges described above,a nd ad ynamicl ikely to impact UK nuclear strategy,i st he growth, spread and general acceptance of BMD as ak ey componento fg lobal security thinking. While BMD is primarily aproject pursued by the US -aclose UK ally -and is theoretically designedt oc ountert he rise of new' undeterrable actors',s uch systems area lso being developedb y aw ide variety of other countries too. 23 While effectiveanti-SLBMsystems remain some wayo ff, the spread of missiledefence capabilitiesa nd technology has already caused visible concern aboutthe potentiali mpact on nuclear-deterrent forces in Moscow and particularly in Beijing, 24 and therei ss trong reason to believe thats uch systems willd iffuse and be adaptedbymoreactorsglobally in the future.AsMalcolm Chalmerspoints out: 'USs uccessi nd eploying effective ballistic missiledefences could also,asa result of inevitable technology transfer, increasingly call into question the viability of ad eterrentf orce basedo ni nflicting unacceptable damagew ith as fewa s eight ballistic missiles launchedf roma single submarine'. 25 This would in fact presentasimilarproblem to thatwhich faced the UK in the late 1970s following the deploymento ft he Galoshm issiledefence shield around Moscow. 26 While the Trident-based system-developed in parta sar esponse to Galosh -w ill very likely be able to overcome any limited defencesinthe shorttomedium term, ever-more effectived efences may presentag rowingc oncern fors uch a small nuclearf orce in the long term. This could be akey factor in determining both when andhow the UK will need to replaceand possiblyupgrade itsstockpile of nuclear warheads,aswell as whether the Tridentmissile canremain viable until the 2040s as predicted.
Af urther significanta spect of this problem is the possibility thatf uture developmentsi nc onventional counterforcet echnologies, particularlyv arious ASWc apabilities, alongside an ew suite of dynamics associatedw ith cyber, could bring into question the security and viability of the UK nucleard eterrent -e specially in ac risis.I ndeed, there is evidence thatA SW capabilitiesa re far bettert han has beeng enerally accepted in the past, and theya re only likely to improvei nt he future. 27 Likewise,t he growth and spread of various cyberweaponsa nd capabilitiesi ncrease the risk of newv ulnerabilitiest hat might be exploited throughout the UK nuclear enterprise, some of which could potentially reduce confidence in the efficacy of the Tridentm issile system, or mightalso compromise the invulnerability and stealth of the submarine. 28 The potential fora na dversaryo ft he UK to discover the patrol area of British submarines or the specifics of the boat, missileo rw arhead through cyberespionage, the possibility of interfering with keys ystems in the procuremento r maintenance phase, or the prospect of lacing targeting or fire-controls oftware with malware, combined with betterASW and BMD capabilities, is clearly aserious issue. Aw orst-case scenario,t herefore, is thata ts ome pointi nt he futuret he UK could be uncertain thati tc ould hold targetsv ulnerable with high confidence, thatits submarines remained secureand safe againstattack, or whether the Trident systemhad beencompromised and would DevelopmentsinBMD capabilities will be keyindetermining Trident's future viability TRIDENT REPLACEMENT AND UK NUCLEAR-DETERRENCE REQUIREMENTS work as planned if required. The needto guardagainstsuch unforeseen challenges to nuclear platforms is ak ey reason why both the US and Russiar etain at riado f nuclearforces.
Lastly,itisimportanttocontemplate whether the threato fd evastating nuclear retaliation (particularly against counter-value targets) will remain crediblea nd viable in an international environmenti ncreasingly characterised by anti-nuclear public opinion. Such sentimenth as grownc onsiderably over thep astd ecade, both domestically in the UK and internationallyt hrough the Global Zero movement, the International Campaign to Abolish Nuclear Weapons, the HumanitarianI nitiativea nd others. While this pressurei su nlikely to be decisivei nU Kd ecision-making,i td oes nevertheless appear to be qualitatively differentfromthe nuclearprotestsofthe 1950s and 1980s. The UK is also the only nuclear-armed statet ob ef ully signed up to the InternationalCourt of Justice, which would place ap articular burden on nuclear-weaponspolicy should aban come into force. 29 As PhilippeSands and Helen Lawp ointo ut in their reviewo f the legal aspects of Tridentrenewal: 'We find it hardt oe nvisagea ny scenario in which the use of Trident, as currently constituted, could be consistent with the IHL [InternationalH umanitarian Law] prohibitions on indiscriminate attacksa nd unnecessarys uffering'. 30 Given thatl ike-for-liker eplacement meansdeterrence by nuclearretaliation (excepti nt he moste xtraordinaryo f circumstances 31 ), it is important to take noteo ft he normative and political environmenti nw hich such policies will be acted out and thatw ill form theb ackdrop forU Kn uclearp olicy and doctrine in the comingdecades.
Long-term UK nuclear-deterrence strategy thereforen eeds to consider The need to guard againstunforeseen challengesiswhy the US and Russia retain a nuclear triad threef undamentalq uestions posedb y these emerging dynamics:first, whether all types of future nuclear threatt ot he UK canbeorare bestdeterred primarily through the threato fc ounter-value nuclear retaliation; second, whetherthe Vanguard-Tridents ystemw ill remain invulnerable, reliable and credible in aw orld where potential UK nuclear adversaries area rmed with increasingly sophisticated missiled efences, ASW weaponsa nd cyber-capabilities; and third, whetherap olicy of threatening nuclear massd estructionc an remain legal, politicallyc redible and effective in thel ong run. As explaineda bove, therea re good reasons to question all of these assumptions given the changes and developmentsl ikely to characterise and reshape the futured eterrence environment.

Assessing Options forUK Strategic Deterrence
Given theq uestions raiseda bove,i t seems sensible and prudentt oe xamine the implications of alternativen uclear futures fort he UK and to contemplate differento ptions within ab roader and moreh olisticf ramework. Accordingly, it makess enset oc onsider three different possible futurep athways: the first, 'businessasusual',CASD andretaliatory nuclear deterrence; thesecond, ablend of nuclear and advanced conventional assetsa llowing form ored iverse deterrence capabilities; and third, a strategy of non-nucleard eterrence. It mayw ell be that the Vanguard-Trident system, CASD and deterrence through the threatofnuclear retaliation remains the besto ption to meett he UK's longterm strategic requirements (as the TAR concluded), but this should not simply be assumed apriori or otheroptions be excluded from the debate.

Domesticand internationalantinuclear public opinionhas grown considerably over the lastdecade
The first,a nd perhapsm ostl ikely, option is 'business as usual',w hereby the UK governmentd ecides to replace the current systemw ith an ew fleet of nuclear-capable submarines armed with Trident SLBMs beginning in the late 2020s, giving the UK anuclear-weapons capability well into the second halfo f this century. This is likely to involvet he retention of CASD and four boats, but it is possible thatCASD mightbeachieved with threes ubmarines, 32 or thatt he UK might adoptam ore' relaxed' nuclear posturew herebyt he one boati sn ot permanently on station and ready to fire, but could be 'surged' into action if required. 33 As has beendiscussedabove, thereare potentially three main problems with this approach: in the first instance, it is not clearthatall future nuclearthreats to the UK will be deterrable purely through the threatofnuclearretaliation, or thatnuclear counter-forcerepresents aparticularly effectiveorpalatable option forUKpolicy againstnew nuclear threats; second, the credibility of the UK nuclear deterrentcould be compromised by the spread of ever-more effectiveb allisticmissiledefences, various types of cyberattack and interference, or advancesi n conventional ASWcapabilities; and third, the political-normativee nvironment of the futurem akes the threata nd/or use of nuclearforcesbythe UK lesscredible. In sum, it is possible thatt he UK might face af uturen uclear threat scenario in which the taboo has beenstrengthened; traditionala dversaries have advanced defencesa nd otherc apabilities; critical systems mighthavebeen compromised; and newn uclear threats aren ot best addressed through retaliation alone. The 'business as usual' approachistherefore premised -c onsciously or not -o na beliefthatinternationalnuclear relations willn ot changes ignificantly from those thatc haracterisedt he Cold War, and thatR ussia and MADw ill remain the centrepiece of UK nuclear planning. This approach willalso likely involvesignificant continuedinvestmentinmodernisingand improving UK nuclear forces over the comingdecades.
As econdo ption is fort he UK to consideramored iverse nuclear and non-nucleard eterrencet oolkit, which might include somet ype of retaliatory ANDREWF UTTER nuclear weapons systema sw ell as a greaters take in missile defence and an advancedc onventional precision-strike capability.This might resemble the 'New Triad' of strategic forces outlined by US PresidentG eorge WB ush in 2002althoughi tw ould be much smaller. 34 The first possibility would be a' diverse deterrence max' posture, wherestrategic competitorss uch as Russia and China would remain at the centreofUKnuclear planning,b ut movesw ould be made to address other nuclear concerns as well. This would see the currentTridentsystem renewed, but augmentedw ith new, advanced conventional capabilities. The second option is a' diverse deterrence lite'p osture, which movesa wayf rom large nuclearp owersb eings een as the mainnuclear threatand raisond'être for UK nuclear weapons, and focuses instead on newand emerging nuclear challenges. This approach could possibly allowt he UK to pursue ar educed nuclear forceeither fewersubmarines,non-continuous patrols, or even some other optionasa lastr esort -w hile enhancing various advancedc onventional deterrence-bydenialcapabilities.
In order to achieve this,t he UK might consideragreaterr ole in US and NATO BMD programmes, and perhaps even an indigenous systemb uilding on the Fylingdalesa nd Menwith Hill radars. Conceivably,t his could involve upgrading the UK'sType 45 destroyers 35 particularly the Principal Anti-Air Missile System, known as Sea Viper 36 -f or a BMD role, building an ew fleeto fB MD ships,oreventhe deploymentofalandbasedcomponentbased on the US Aegis Ashores ystemp lannedf or deployment in Europe. 37 The UK could also consider whethera dvancedc onventional precision-strikew eaponry could playa counter-forcer ole againsts pecific types of threats in this futuree nvironmentpossibly based on aconventionallyarmed The UK mightconsider playing agreater role in US or NATO BMD programmes Trident D5 missile. Alternatively,t he UK could offer to hosts uch weaponsf or the US and/or as parto faw iderN ATO policy.Such amix of forces would allow am orefl exible deterrence approach that could be tailored to differentt ypes of nuclear challenges, andi tw ould also retain the ultimatet hreato fn uclear retaliation in extreme circumstancesalthough this would be reducedu nder ap ostureo fd iverse deterrence lite. Diverse deterrence maxw ould be the mostc omprehensiveo ption fort he UK, but would also necessarily be the most expensive. 38 Afi nal option might be to rely entirely on conventionala nd advanced conventionalw eaponsf or UK strategic deterrence, andthereforetounilaterally disarmf or strategic rather than political reasons. Thisi sc learly the mostr adical option, and would probably have to be basedo nam ilitary calculation thatt he 'traditional' Russian nuclear threatshould no longer be central to UK planning. That said, the idea has already been raised in a2014 UK Defence Select Committee report,which noted that'it is possible to foresee an environment in which the core role of nuclear deterrence -t op rotect as tate from attack -i sa chieved by the deploymento fa dvancedc onventional weapons, providing both offensivea nd defensivec apability'. 39 An on-nuclear strategic deterrentp osturew ould rely on amixtureofmissile and air defences, conventionalp recision-strikec ounterforcecapabilities as well as an upgraded conventionalm ilitary. Such am ove would clearly have certain political and diplomatic benefits -p articularly for the NPT-a nd could seet he UK hailed as a' disarmamentc hampion' (although the impact of this is debated). 40 The UK would, presumably,s till be coveredb y the US extended-deterrence umbrella committed to NATO shoulda ny serious 'Diverse deterrence max' would be the most comprehensive, but mostexpensive, option strategic threatr e-emerge thatc ould not be metw ith conventionalf orces. Interestingly,ad ecision in favour of non-nuclear deterrence would not necessarily mean scrapping the next generation of nuclear-poweredS SBNs as these could offer ap latform fora futureU Kc onventional global promptstrikecapability,althoughthis remainsa complicated optiongiven the difficulties of discriminating betweenanuclear and non-nuclear launch.

Having theRight Debate about UK Nuclear Weapons
The decision to replace the currentfleet of Vanguard submarines, and therefore the UK's nuclear-deterrentc apability, has driven adiverse and heateddebate, but this discussion has beenl imited and focused primarilyo ns hort-term political and economicdynamics rather than long-term thinking aboutt he requirementso fn uclear deterrence. UK nuclear strategy remains based on the assumption -c onsciously or not -t hatas mall,r etaliatory nuclear forcec apableo fd eterrence through punishment will be sufficientt om eet then uclear threats of an uncertain future.H owever,i ti si ncreasingly unclear thatsuch asituation will prevail: first,t he natureo ft he possible future nuclear or existential threattothe UK is unlikely to be homogeneous, and may required ifferentt ailoreda pproaches and types of deterrence tools; second, it is not clear thatsuch asmall deterrent forcew ill remain invulnerable, credible and alwaysa ble to inflict unacceptable damagei naf uturee nvironment dominatedb ys ophisticated defences, newv ulnerabilities associated with cyber,a nd advancesi nA SW technologies; third,i ti sb ecoming progressively difficult politically,l egally andd iplomatically to rely on ap olicy of inflicting massc asualties through an uclears trike, and this will call into Disarming forstrategic rather than political reasons would be the most radical option Notes 1T rident-ormorecorrectly,Trident II (D5) -isthe name of the ballistic missile leased by the UK from the US that'delivers' UK nuclear warheadsto their target,but the term 'Trident' is also often usedtorefer to the whole UK nuclear systemthatcomprises the Vanguard submarines, the warheads andthe associatedUKnuclear-weapons infrastructureatAldermaston, Coulport and Faslane. question the credibility of the UK to go through with such threats. Taken together,t hese dynamics suggest am ored iverse and complex future deterrence environment, ando ne that does not currently appear to featurein the UK nuclear debate.Asaresult, it is essential thats ome serious thoughtb e given to who the UK is and will be trying to deter, howt hese potential threats canb ed eterred,a nd with what, and the extent to which thisp osturec an remain credible and efficacious, before embarking on ac ostly,i ntrinsically inflexible, multi-decade commitment to the next generation of nuclear weapons. 