London — European readiness to follow the United States lead in Western defense depends heavily on two key factors: quality and consistency in American policy. The rift now opening up between the US and its allies on nuclear policy is largely the result of a growing feeling on this side of the Atlantic that the Reagan administration has so far failed to appreciate this fact.
In London and other West European capitals public disputes between US Secretary of State Alexander M. Haig Jr. and other high US officials are being seen as a threat to the bond of confidence that has kept America and Europe more or less ''on track'' since World War II.
Almost equally damaging is the open row that has developed between Mr. Haig and British Foreign Secretary Lord Carrington over Middle East policy.
Carrington speaks for the 10-nation European Community (EC) when he casts doubts on the long-term future of the Camp David peace process and edges toward acceptance of the Saudi Arabian peace plan. Haig's semipublic warning to Carrington, suggesting that he ''cool it,'' has provoked tight-lipped anger not only in London but also in the capitals of other EC countries.
A massive effort is being made to keep the anger buttoned down, because it is widely recognized there that if there were a verbal transatlantic battle over Middle East policy, the Western allies would appear in disarray even more than they do today.
European governments were thrown into confusion by conflicting statements last week by Mr. Haig and US Defense Secretary Casper Weinberger on whether there is a NATO contingency plan to detonate a nuclear ''warning blast'' in case of attack.
In West Germany, official sources at first said Mr. Haig was correct in saying such a plan existed. But when Mr. Weinberger contradicted the secretary of state, the same source said Mr. Haig had it wrong.
European diplomats in Brussels, headquarters of NATO, agreed unanimously that citizens would be horrified at the use of nuclear weapons in a ''demonstrative way.''
Said one NATO military adviser: ''Such weapons cannot be used without a high risk of escalation to a full-scale nuclear exchange. The Americans are making life hard for West European governments by unsettling the very people who vote those governments in and out of office.''
In London, British Defense Secretary John Nott accused the media of overdramatizing the statements by Haig and Weinberger, but Labour opposition defense spokesmen said they were aghast at arguments developing in Washington.
A large part of the alarm caused in Europe by the uncertain signals emanating from the Reagan administration is a result of the growth of the so-called peace movement of antinuclear protesters.
The campaigners have been focusing on NATO's plans to install theater nuclear weapons on the soil of European members of the alliance. They have also accused the Reagan administration of dragging its feet over steps to promote arms reduction negotiations with the Soviet Union.
Washington's present disarray enables much of the antinuclear campaign to base its argument on the alleged lack of competence of President Reagan and his advisers.
In the British Foreign Office, it was hoped that one year after Mr. Reagan was elected he would have produced nuclear policies for Europe sensitively constructed to meet the main arguments of the protesters.
Part of the problem is that West European governments down the years have not actively encouraged public discussion of nuclear matters. Now public opinion is apt to be under pressure from opinionated protesters, many of them basically anti-American.
President Reagan's failure to produce unity of nuclear policy among his own advisers is making it relatively easy for the peace movement leadership to score points off the US.
Mr. Haig's role in this process is alienating him among those Europeans who so far have been apt to trust the secretary of the state rather than Mr. Weinberger.
On the one hand, he is seen as a relative moderate with firsthand experience of the European scene. On the other, he is currently perceived as a man who lectures Europeans - on the need to accept the American lead in defense matters and to soft-pedal attempts to bring the Palestinians into the Middle East peace process.
A measure of the dismay among Europeans sympathetic to Washington's broad intentions was given editorially by the London Sunday Times. Under a heading ''Lest We Forget, America is on Our Side,'' the paper deplored the Reaganites' ''confusion and tactlessness.''