NINE countries are believed to have nuclear weapons. On July 18th Britain will decide whether it wants to remain in that club, when its MPs debate whether to renew the country’s Trident nuclear deterrent. Theresa May, the new prime minister, has said it would be “sheer madness” to give it up, and the vote is expected to pass easily. Perhaps 150 of Labour’s 230 MPs will vote in favour of the plan, rebelling against their leader, Jeremy Corbyn.

The House of Commons approved in principle the retention of a nuclear deterrent in 2007. A review in 2013 reaffirmed that “like-for-like” replacement of the four submarines that carry the missiles represented the best and most cost-effective way to do it. Parliament will now decide whether to approve the spending of £31 billion ($41 billion) over 20 years to replace the four Vanguard-class subs, which will wear out within a decade.

Trident’s detractors argue that a lot has changed since the programme was approved in 2007. For one thing money is tighter. Around one-quarter of defence spending on new equipment procurement will be on submarine and deterrent systems by 2021-22. There has also been a surge in support for independence in Scotland, where the submarines are based. It is unlikely that the government would choose to site the capability north of the border if the renewal process began again now, says William Walker of St Andrew’s University. The Scottish government opposes the plan; almost all of the 59 Scottish MPs at Westminster are expected to vote against it (though polls suggest that public opinion in Scotland is more mixed). If Scotland were to become independent—now more likely because of Brexit—Britain could well have to relocate its subs, at further expense.

Critics also say Trident relies too much on a single naval platform (America has air, land and sea options), and that improved ballistic-missile defences and the future use of underwater drones and cyber warfare could threaten the subs’ security. Yet land-based ballistic missiles are vulnerable to attack, and arming aircraft with nuclear-tipped cruise missiles permanently aloft carries a significant danger of nuclear accident and is much more expensive. The cut-price option of building three submarines rather than four would be a false economy, undermining the principle of “continuous at-sea deterrence”.

The vote comes at a time when few in Britain are minded to dial down the country’s defence capabilities. Mrs May has cited Russia’s renewed belligerence as one justification for updating Trident. And Brexit has left the country, and its allies, shaken. Britain’s partners would be sensitive to signs of more isolationism, says Malcolm Chalmers of RUSI, a think-tank. Britain has the largest defence budget in Europe; maintaining nuclear capabilities shows that it is still committed to NATO. “Our allies would not understand if we chose this moment to give up our nuclear weapons,” Mr Chalmers says.

The vote is also linked to Britain’s image of itself. Last year a strategic review boosted defence spending, as part of an effort to restore Britain’s standing as a military power after years of cuts. Trident is part of that. Though it is expensive and imperfect, most MPs, and their constituents, believe it still helps to make Britain safe, and is a force for stability—something of which it has had precious little in recent weeks.