IT is not often that the French President, the British Prime Minister and the German Chancellor write to a newspaper. But Jacques Chirac, Tony Blair and Gerhard Schroeder have just put their names to a joint appeal in the New York Times to the United States Senate to ratify the treaty to ban nuclear tests. Their demarche is a sign of how seriously the rest of the world takes the probability that the Senate would refuse to ratify the Comprehensive Test Ban Treaty if a vote is held as scheduled this week. Facing the prospect of failing to muster the necessary two-thirds majority on the issue, President Bill Clinton says he is open to delaying the vote. What he refuses to accept is a Republican demand that he agrees to put the whole matter off until a new Congress takes office in 2001. But the Senate Majority Leader, Trent Lott, immediately dug his heels in by insisting that the White House must agree to postpone consideration of the treaty until Mr Clinton's successor takes office. Nuclear weapons cannot be dis-invented, but the dream of limiting their development and spread has taken on particular lustre since the end of the Cold War. The post-1945 situation in which they were seen by both the West and the Soviet Union as the means of preventing the other side from triumphing now belongs to history. Instead of providing the ultimate rampart of defence, nuclear arms are seen as threats to world peace whose possession should be as tightly restricted as possible. The fact that they have not been used since the atomic bombs dropped on Japan does not lessen the need to prevent them being deployed by rogue states or in regional conflicts. This gives particular force to Mr Clinton's appeal to the Senate to take the issue 'out of politics', particularly given the popular support for the treaty shown in opinion polls. Whether the bulk of Republican senators want to go down as being responsible for the first defeat of a major international arms treaty in more than 70 years remains open to question. Apart from anything else, it could look bad in the coming presidential and congressional elections. But while not all would go so far as the hard-line senator who calls the treaty 'nothing short of unilateral disarmament', its critics insist that the pact cannot be verified and that it would weaken the ability of the United States to test and maintain its own nuclear arsenal. Those concerns certainly have some validity. As Saddam Hussein has shown, a determined ruler can always hamper international inspection. But such concerns are outweighed by the fashion in which the treaty opens the way for effective monitoring in the air, at sea and on land with a force of international agents empowered to carry out spot checks of suspicious activities. In all, 154 countries have signed the treaty so far. But the position of the United States in pushing forward the non-proliferation process is pivotal, not only because of the size of its nuclear arsenal but also because the treaty will only take effect if the US is among the nuclear-capable countries that have to give ratification for it to come into force. As Mr Chirac, Mr Blair and Mr Schroeder warned at the weekend, a failure by the US to agree to the treaty would act as a great encouragement to proliferators. Any country planning to develop its own nuclear arsenal would only have to point to Washington as an answer to critics. Pressure on the two newest nuclear powers, India and Pakistan, to halt their programmes would be that much more difficult to apply. Russia and China would feel less need to sign the treaty - not to mention North Korea. Europe's three leading powers, France, Britain and Germany, have all ratified the treaty (France after conducting its controversial 1996 tests in the South Pacific). A split between their position and that of the United States would, in the words of the three leaders, 'expose a fundamental divergence within Nato'. The alliance's new Secretary-General, George Robertson, emphasised the point by telephoning Senator John Warner, the senior Republican senator on military matters, with a last-ditch appeal for backing. After maintaining its cohesion during the Kosovo crisis, the last thing the alliance wants is a split on an issue where Washington will be seen to be paralysed by partisan domestic political considerations. The move to put the lid on nuclear weapons is eminently worthwhile. It is a process in which successive US administrations have played a key role. That momentum must be maintained. Whatever the procedural manoeuvres that may be needed in Washington, the Senate owes it to the world to ratify the treaty - if not this week then as soon as possible. Shelving it until 2001 would send a dangerous signal to the world, and would be a dereliction of duty by the planet's sole superpower.