A perennial question faced by the major nuclear powers is how many and what kinds of nuclear forces are necessary to maintain an effective, credible deterrent. Contending assessments of security threats, as well as differing strategic concepts, have traditionally informed the debate. Yet, the tug-and-pull of domestic politics can also play a significant role in determining a nation’s nuclear policy.

That’s certainly the case in the United Kingdom today. A decision on the future of Britain’s nuclear deterrent could depend as much on the dynamics of intracoalition and interparty politics—and a referendum on Scottish independence—as it does on strategic analysis.

Missiles and Submarines

The United Kingdom is one of the five nuclear-weapon states officially recognized by the 1968 Nuclear Nonproliferation Treaty. At one time, it fielded a wide variety of nuclear-capable weapon systems. (According to Professor Sir Lawrence Freedman, these included longer-range bombers, fighter aircraft and maritime helicopters capable of delivering British-produced nuclear weapons; as well as short-range land-based missiles and artillery able to fire U.S. nuclear weapons under a dual-key arrangement.) But by the late 1990s, Britain had phased out all of its air-delivered and land-based nuclear-weapon systems as part of a post–Cold War adjustment that led to a substantial reduction in the total number of deployed nuclear weapons.

Today, the UK’s nuclear force consists of four Vanguard-class nuclear submarines, each armed with up to sixteen Trident D-5 ballistic missiles. The UK builds its own submarines and nuclear warheads; the missiles are purchased from the United States under the terms of a sales agreement that dates back a half-century. With four submarines in service, at least one can always be under way and on patrol. British government officials have long regarded “continuous at-sea deterrence” as essential to maintaining a credible deterrent, because it ensures that at least a portion of the nuclear force is likely to survive any attack and still be capable of mounting a retaliatory strike.

The UK will soon be forced to decide on a successor to the current Trident system. Its ballistic-missile submarines were built between 1986 and 1999, and are expected to reach the end of their service lives starting in the 2020s. Given the long lead times required to design, construct and commission new submarines, serious work will need to begin soon if they are in fact to be replaced.

Coalition Politics

A final decision on what to do as the UK’s Trident submarines age out has been purposely put off until 2016, after the next general election. Nevertheless, the positions of the major political parties are already well established. In 2006, Prime Minister Tony Blair’s Labour government decided to “maintain our deterrent system beyond the life of the Vanguards with a new generation of ballistic-missile-carrying submarines.” Conservative leaders—including current prime minister David Cameron—have similarly advocated “like-for-like” replacement of the existing UK ballistic-missile submarines.

The junior partners in the present ruling coalition—the Liberal Democrats—have taken a different view. They have for some time argued that like-for-like replacement of the current Trident boats would be inordinately expensive. Moreover, they maintain that a continuous at-sea deterrence posture—and thus a four-submarine fleet—is no longer necessary in the post–Cold War security environment.

After the May 2010 general election, the Conservatives and Liberal Democrats struck a compromise of sorts while forming their coalition. They agreed, inter alia, that the new government would remain committed to the maintenance of Britain's nuclear deterrent. (To this end, some initial work on replacing the Vanguard-class submarines has been undertaken and funded.) However, the coalition agreement also stated that “the renewal of Trident should be scrutinised to ensure value for money.” Additionally, the Liberal Democrats were explicitly allowed to continue making the case for alternatives.

Last week, the Cabinet Office released the long-awaited results of an internal review on likely alternatives. The written report examines the costs and strategic implications of various weapons systems, including stealthy or supersonic cruise missiles that could be launched from large aircraft, fast jets, surface ships, or attack submarines. It also addresses various “force postures,” ranging from the current practice of having a portion of the force always on full alert to more relaxed levels of readiness.

While the report made no specific recommendations, it clearly makes the case that replacing the current ballistic-missile submarines with new ones would ultimately be the most affordable and most feasible means of preserving an independent nuclear deterrent. In large part, this is because the UK already possesses the infrastructure, expertise and experience for operating ballistic-missile submarines. The UK also benefits enormously from close collaboration with its American ally in sustaining its Trident missiles and their associated equipment. On the other hand, developing an entirely new delivery system, as well as a compatible warhead, from scratch would prove costly, time-consuming, and highly risky.

The political reaction to the report suggests it may have had a subtle, yet discernible impact on the debate. The Conservatives predictably saw the review as validating their current position on like-for-like replacement. As for the opposition, the report bolstered Trident supporters within Labour’s ranks and helped facilitate a public reaffirmation of Blair’s earlier commitment to a new generation of ballistic-missile submarines. Shadow defense secretary Jim Murphy responded that “Labour has always said that we are committed to the minimum credible independent nuclear deterrent, which we believe is best delivered through a Continuous At Sea Deterrent. It would require a substantial body of evidence for us to change that, but this review does not appear to offer such evidence.”

With the prospect of finding more affordable alternatives to Trident now apparently off the table, the Liberal Democrats refocused their talking points to emphasize the financial savings that could be achieved by cutting the number of submarines from four to three, or even two—which of course would spell an end to continuous at-sea deterrence. Austerity measures adopted by the current Government have already hit the British defense establishment hard. The British army faces an approximate 20 percent reduction (from 102,000 to 82,000 regular personnel) over the next five years; the Royal Navy has already lost both personnel and ships, including its sole remaining aircraft carrier, Ark Royal. Nuclear forces have so far largely escaped the budget axe. The Liberal Democrats obviously plan to tout the need to find savings in the nuclear account as well.

Finally, as if differences among the political parties on the future of nuclear deterrence were not enough, regional politics have also entered the mix. Scotland is due to hold a referendum on independence next year. Two major facilities associated with the Trident force—Faslane and Coulport—are located in Scotland. Leaders of the Scottish National Party have vowed that if Scotland does indeed gain independence, the nuclear subs must leave. The options for relocating are limited and the costs of doing so are astronomical.

The debate over the UK’s nuclear future will no doubt continue through the next general election. The Liberal Democrats have promised to address the issue at their party conference in September. But with the two major parties still expressing support for an independent nuclear deterrent based on continuous at-sea deterrence, the next British government is likely to make, at most, only modest changes to Britain’s nuclear forces. Labour’s shadow defence secretary has suggested, for example, that the total number of missile and warheads carried on board submarines could be reduced. As for next year’s Scottish referendum, the smart money says that the independence movement will fall short, though more for economic reasons than any concern about the future of the UK’s nuclear deterrent.

But if either of the above two assumptions turns out to be wrong, all bets about the UK’s nuclear future are off.

The American Connection

The ultimate outcome of the UK domestic debate on Trident replacement also has implications for the United States. Anglo-American cooperation on nuclear weapons has been close ever since British scientists participated in the Manhattan Project during the Second World War. The Kennedy administration’s decision to sell Polaris sea-launched ballistic missiles, later updated to include Trident missiles, has been a centerpiece of the U.S.-UK strategic relationship for the past fifty years. If the United Kingdom ultimately proceeds with like-for-like replacement of the existing Trident system, close cooperation with the United States will continue to be essential.

However, according to at least one press account, unnamed senior American officials have quietly expressed doubts about the wisdom of the UK’s continued investment in the nuclear forces, especially as the British Army—which has fought alongside US troops in the Balkans, Iraq and Afghanistan—is being pared to the bone. As if to quell any thoughts of waning American support, Secretary of Defense Chuck Hagel, in the presence of visiting UK defense secretary Philip Hammond, recently congratulated the Royal Navy for its “steadfast maintenance of its submarine-based nuclear forces and their continuing round-the-clock patrols.” Hagel then added, “I strongly support the United Kingdom's decision to maintain an independent, strategic deterrence.”

He was right to do so.

Lt Gen Frank G. Klotz, USAF (Ret.) is a senior fellow at the Council on Foreign Relations in Washington, DC, and the former commander of Air Force Global Strike Command, which has responsibility for all U.S. nuclear-capable bombers and land-based missiles.