“Cock your ear to how tentative and apologetic the argument for a new generation of British nuclear missiles is becoming,” wrote Matthew Parris on Saturday. Not so. The tentativeness comes from opponents of a British nuclear deterrent.

Anti-nuclear campaigners were once driven by apocalyptic foreboding. (The Tories, wrote E.P. Thompson in 1983, are “not a party which can be returned to government without risk to our lives”.) These days the rhetoric is tamer. Opponents of replacing Trident even implicitly concede the Cold War argument for an independent deterrent by complaining that there is now no obvious state against whom our weapons would be targeted. They conclude that Tony Blair is unreasonably swayed by Labour’s electoral disasters of the 1980s.

Certainly, Mr Blair is anxious to inoculate Labour against the perception of weakness on defence. That is only sensible. Barring one aberrant decade, Labour is a party of nuclear defence. Labour governments initiated Britain’s nuclear weapons programme, and modernised the old Polaris fleet. It is historically apt that new Labour should inaugurate new nukes.

The Labour MP Michael Meacher declares that Trident “answers no threats that we currently face” — as if today’s threats should determine defence planning to 2040. Defence policy must anticipate remote contingencies. Even our present knowledge of the capabilities of despotic regimes is limited. (Consider the intelligence failures over Iraq’s WMD.) Our nuclear deterrent is thus an insurance policy. We cannot predict the threats to us in the middle of this century, just as 30 years ago we did not foresee Argentina’s invasion of the Falklands and Saddam Hussein’s annexation of Kuwait.

Not in all cases, but in some, our independent deterrent may act as a political counterweight, causing a potential aggressor to think again. That benefit is necessarily speculative, but the costs are known. The capital cost will average 3 per cent of the defence budget. (The current Trident fleet, like its Polaris predecessor, was introduced on time and within budget.) The diplomatic benefits of renouncing our nuclear capability, conversely, would be trifling and the costs incalculable. It would be an irreversible step taken in an uncertain world where the worst of states — Iran and North Korea — appear intent on acquiring nuclear weapons. Those states will scarcely be swayed by a diplomatic gesture by the UK. Nor will any current nuclear powers, whose policies are geared to their own perceived strategic interests.

The PM has taken a contentious decision in the short term, so that his successor need not. It is the stuff of leadership, and it is right.

Times Recommends

• It’s the beginning of the affair
• Afghanistan: still a price worth paying
• Heroes of the downtrodden