UNITED States President Bill Clinton has made another foreign policy mistake. But this time he has done it coldly and deliberately. He has no better excuse - to judge by the comments of his White House staff - than the desire to humiliate the British for helping George Bush in the 1992 election campaign and to buy favours from powerful senators of Irish descent. Recommending a visa for Gerry Adams, the smooth-talking leader of Sinn Fein (the political wing of the Irish Republican Army) is the kind of cheap trick a politician can promise when running for the presidency. But it is a promise he should jettison whenhe gets into office. Would-be chief executives can offer gimmicks to win the Irish-American vote. Real presidents have to exercise power responsibly. Letting the Sinn Fein leader on to American soil, and American television, will win the approval of important Irish lobby figures, such as senators Edward Kennedy and Patrick Moynihan - who just happen to chair committees vital to the passage of Mr Clinton's health and welfare reform programmes. But it will not end the violence in Northern Ireland. It will not save lives. It will not help swing the political balance against the minority group Sinn Fein and in favour of the peace-making Social Democratic and Labour Party (SDLP). In Catholic areas this party draws the majority of the Catholic vote. It is not as though Mr Adams had spoken the magic words: ''I renounce violence.'' He did not. State Department officials are reported to have been adamant that he merely repeated his usual ambiguous, mealy-mouthed references to innocent civilians. Instead, a man who remains committed to the murder of civilians he feels are somehow not innocent is being given the freedom of the American media. The world has no more powerful platform. And this courtesy of a weak and desperate president who would help the spokesman of terrorists for the sake of domestic political gain. Some seek to justify Mr Clinton's decision to deal with terrorists. They draw a comparison with the Israeli Government's decision to talk to Yasser Arafat and argue that the time is right for such generosity. Only courageous attempts to put past hatreds in the past, and risk bitter opposition from extremists on both sides, will bring the chance of peace, they say. The difference is that the Israeli Government has been trying to solve its own problem, not giving someone else's problem free publicity. The irony is that John Major has already made peace overtures to Sinn Fein, incurring criticism from those who fear he is selling out the Protestants of Northern Ireland. Mr Adams, far from seizing the challenge, has been offering weasel words in response, undermining the credibility of both the Prime Minister and the SDLP. And the IRA has continued killing. It does not want peace. It wants Britain out and it wants Ulster to remain under the rule of the gun. Its gun. Mr Clinton has shown up his own shallowness and political vulnerability in acting as he did. But above all he has shown cynicism. That is not the leadership the world expects of a US President.