The real issue is national missile defence (NMD), the plan for a computer-controlled early-warning shield around the whole of mainland US airspace to detect and intercept ballistic missiles from terrorists or rogue states. The 1972 treaty is an accidental obstacle to this science-fiction delusion. It is incumbent on America's friends to say whether they are for or against it. Tony Blair will say nothing.

He should be against it and, privately, he probably is. The NMD project is misconceived, for the relatively trivial reasons that it cannot work and that it will, therefore, waste vast amounts of money that the richest country in the world could put to better use. It is only worth having if it can be 100 per cent sure of detecting hostile missiles - and not confuse them with, for example, civil airliners. That is technically impossible. Yet Mr Blair will say nothing.

As Mr Bush rightly observed yesterday, the suicide attacks in America in September were a vivid demonstration of the reality of the threat from terrorists and rogue states. But anyone with an ounce of intelligence can see the other significance of the destruction of the twin towers, which is that the attacks were launched from within US airspace and would not have been prevented by NMD. Yet Mr Blair will say nothing.

The more serious objection to the space-age shield is that it is based on the dangerous belief that countries can bat back missiles instead of trying to stop them being launched in the first place. The repudiation of an old treaty does not matter in itself. What matters is that trying unilaterally to construct a missile shield weakens the culture of international arms restraint. Yet Mr Blair will say nothing.

Moreover, the timing of Mr Bush's announcement could hardly have been worse. Yesterday's suicide attack on the Indian parliament draws attention to the need for closer co-operation between nuclear-armed and nuclear-capable states. If, as seems likely, the terrorists were motivated by the desire to end Indian rule in Kashmir, itself often allied with extreme Islamicism, the omens for relations between the Subcontinent's nuclear powers are poor.

The symbolism of the US going it alone, against the objections of Russia, is just what the world does not need. At a time when it is harder than ever to persuade India and Pakistan to join international efforts to restrain nuclear proliferation and ban testing, Mr Bush's unilateralism is deplorable. Yet Mr Blair will say nothing.

The go-it-alone mentality of the Bush administration has resurfaced all too quickly after the coalition talk of the campaign against terrorism. The failure of the US to support the Kyoto treaty on global warming, or the efforts to set up an international criminal court, reveal the President's true colours. And they are likely to show more vividly when mixed with US triumphalism over the rapid collapse of the Taliban.

The Prime Minister clearly intends to endorse the US plans for missile defence when he has to and to grant permission for the US to upgrade its early-warning facilities in this country. He should do neither, and he should exert what influence he has on the American administration to bring it to its senses. Sadly, he seems determined to shirk this responsibility.

But as with so many twists of diplomacy since September 11, Russia may turn out to gain from the move. The only danger, and that is for Putin personally, not for Russia, is that it jeopardises his standing with Russians already suspicious of his policy of cosying up to the US. The biggest loser from the US's move is Colin Powell, the Secretary of State, who had staked the past month's work, and his standing within the Bush Administration, on the argument that the US should stay within the treaty.

President Bush, who told congressional leaders on Wednesday that the US would quit the treaty in six months, has argued since he took office that it was a "Cold War relic". While Russia was prepared to stretch the treaty to allow the US to test a missile defence system, the deal-breaker was that it wanted the right of veto over every single test. Bush refused. Construction of the missile defence system begins in Alaska in April when the snow melts.

In purely military terms, Russia could do well out of this, as its calm, even friendly, response to the announcement shows. It reckons it could overwhelm any missile defence system with the sheer number of its warheads. The move may also release it from its obligations to cut nuclear arsenals under the Start II arms reduction treaty, said Dmitry Rogozin, head of the parliamentary foreign affairs committee and a supporter, in general, of Putin.

In a remark, which nonetheless attracted scepticism, he also suggested that Moscow might respond by placing multiple nuclear warheads on missiles. The reason for scepticism is that Russia is constrained by lack of money, also the driving force behind its desire to cut stockpiles by two thirds, as Putin agreed with Bush last month.

But even if the move is insignificant - possibly even helpful - in military terms, it does turn up the heat on Putin. He is already perceived as being well ahead of public opinion in his warmth towards the US. Now he has received what many Russians will regard as a rebuff. So has General Powell. He lost this central policy battle to Donald Rumsfeld, the Defence Secretary, when Condoleezza Rice, the National Security Adviser and an advocate of many of Powell's policies of negotiation and restraint in the past month, finally sided with Rumsfeld.

European governments, which have often found Powell the most congenial of the Bush team in the past three months and fear the move reflects the rise of American unilateralism, will have taken note of this power shift within Washington. It shows every sign of persisting.

Mikhail Kasyanov, the Prime Minister, said he regretted the decision, which was "a cause of annoyance". He said that other nations had cause for concern but Russia had no fears for its security. Vladimir Lukin, a former Ambassador to Washington and now a deputy in the Russian Parliament, said: "Russians were just beginning to believe they could have a strategic partnership with America . . . the US has shown it will do exactly what it wants whenever it wants without taking our opinion into account."  Many in Moscow still regard the ABM as the cornerstone of arms control rather than an outdated relic of the Cold War, and are concerned that the "shield" will undermine Russia's nuclear deterrent. But diplomats said that Mr Putin had no choice but to accept the decision as he had invested too much of Russia's prestige, and his own, in the new alliance with the US.  President Bush insisted that America's unprecedented step in withdrawing from an arms control treaty would not jeopardise Russian security or Washington's links with Moscow. Giving notice that the US would quit the ABM treaty in six months, Mr Bush promised future gains for Russia. He insisted that he was forced to "move beyond" the ABM treaty because it was preventing America from developing the "Son of Star Wars" missile shield he had promised in his election campaign. Mr Bush said that America and Russia were already well on the way to developing a new strategic relationship that would outlast both his and President Putin's political careers.