Hardly: it makes at least a gesture towards shrinking the deployment of missiles; Britain has helped to devise some of the more imaginative ways of trying to persuade countries to give up nuclear ambitions; it represents no change of policy.

Much more damaging to anti-proliferation efforts is the US pact to help India with nuclear power. That unnecessary offer, born of America’s new desire to court India, might have been redeemed if the US had pressed for tough terms. Instead, its concessions grew more egregious in the past six months; the version passed on November 16 by the Senate overturned US policy and was an assault on arms control efforts.

There is no question that the 1970 Nuclear Non-Proliferation Treaty (NPT) is under siege; the question is whether it can be bolstered enough to keep it in force. It has been signed over the years by 187 states, more than any other arms control treaty, including the five countries that have declared that they have nuclear weapons: the US, Russia, China, Britain and France.

Just before the Prime Minister’s statement on Trident, Hans Blix, the former United Nations weapons inspector, took aim at the five declared nuclear weapons powers. He argued that they could not keep demanding of other countries that they forgo weapons, under the terms of the treaty, without complying with their own obligation eventually to rid themselves of nuclear weapons.

It is hard to treat Blix as an impartial figure since the Iraq war, which he opposed (with reason) on the ground that inspections had found no trace of weapons of mass destruction. Since then, his comments about the US and Britain, in particular, have seemed both caustic and weary.

His attack this week is too narrow. There are many reasons why the NPT is fraying, and the retention of nuclear weapons by those that have them is less provocative than some, in that it has not changed. There are, after all, no firm dates on their obligation to disarm. The treaty is written in terms that assume its own success, not — as has happened — that proliferation suddenly surges ahead.

New strains have come from the acquisition of weapons by India, Pakistan and, it is assumed, by Israel, none of them signatories. The US has gone to great lengths to make exceptions for each — and to rule out any discussion of Israel’s capability, a position that may not be tenable forever if the NPT is to hold together.

Blix is right that the central bargain underpinning the NPT, between the “haves” and the “have-nots” is coming unstuck. But the reasons are much broader than the recalcitrance of the five nuclear powers. Their promise under the treaty to help countries to get nuclear power, if they forgo weapons, has become less valuable, now that the skills are much more easily acquired, as Libya, Iran, North Korea and Iraq in 1991 have shown.

At the same time, the attractions of having nuclear weapons may have grown, for many countries, either as a strategic calculation, or as an expression of national pride.

So the price of persuading countries not to get weapons is rising. The NPT is deceptively neat, in implying that the price can be met by a pure nuclear exchange: access to nuclear power in return for giving up weapons.

That was barely the case when it was written, and for many countries, it is now too paltry an offer. The result is that the NPT has been bolstered by all kinds of deals with waverers that purport to be unrelated, but are not: on trade, on membership of Nato, and so on. As one Foreign and Commonwealth Office official dealing with Iran put it, “it is like buying the same carpet again and again”. That was the inevitable evolution of the NPT, as help with civilian nuclear power can be delivered in a year or two, but persuasion not to develop weapons must continue in perpetuity.

The worse damage the US has done to the treaty is to accept India’s possession of them without demanding participation in non-proliferation efforts — and to offer it civilian help that could be diverted to a weapons programme.

It is spuriously simple to argue that if the five weapons states in the NPT gave up their arsenals, it would do much for anti-proliferation efforts. It is entirely fair, though, to criticise the US for its short-sighted and destructive offer to India.

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