Washington — Europe aches to be reassured that America won't start a nuclear war - and wage it on the European continent. The tension has reached a point where it has caused a diplomatic storm, turned a casual presidential comment into a global incident, and incidentally helped displace the top military officer of the US National Security Council staff, Maj. Gen. Robert L. Schweitzer.
What exactly did the President say? In answer to a question from American newspaper editors in Washington Friday, he replied: ''I could see where you could have an exchange of tactical (short range) weapons against troops in the field without them bringing either one of the major powers to pushing the button.''
If it makes America and the Reagan administration more aware of the supersensitive antiwar sentiment building in NATO countries - the kind that assembled a quarter-million protesters in Bonn, West Germany, this month - many think the incident will be constructive. It is already having effect.
President Reagan, flying to Cancun, Mexico, issued a departure statement saying that the US opposes the use of nuclear weapons at any level.
The incident provoked a sharp exchange between Soviet President Leonid I. Brezhnev and Mr. Reagan. The former interpreted the President's offhand comment as implying that the US is willing to let Europe be a nuclear battleground.
Mr. Reagan, in his statement made en route to Cancun, sought to turn the argument against the USSR. He said: ''The essence of US nuclear strategy is that no aggressor should believe that the use of nuclear weapons in Europe could reasonably be limited to Europe.'' This was a warning to Russia and the world that the US will counter any Soviet nuclear thrust.
The origin of the international dispute had elements of the absurd. The President's comment on Friday was accepted by the American press as being so casual - not to say platitudinous - that nobody used it.
The comment rested there till it started a firestorm in Europe, where US Defense Secretary Caspar W. Weinberger, at a meeting of NATO's Nuclear Planning Group at Gleneagles, Scotland, tried to put it out.
Though not directly connected, the commotion probably weighed in the decision of Richard V. Allen, chief of the National Security Council staff, to relieve General Schweitzer of his White House duties, and send him back to the Army.
General Schweitzer, in a speech which he said he thought was off the record, declared the Soviet Union now has nuclear superiority in all three sides of the nuclear triad (air, land, and sea), an assessment not shared by the administration. He spoke of a ''drift toward war,'' said the ''Soviets are on the move; they are going to strike,'' and said that the US is in the greatest danger in history. He could hardly have picked a more sensitive time. Mr. Allen relieved him, with regret, saying that the general had broken a rule against talking without advance clearance.
The deeper problem as seen here is the matter of President Reagan's and America's credibility abroad. So long as the arms race was accompanied by disarmament negotiations, fears were controlled. But after seven years of bargaining under three presidents, the Senate shelved the SALT II treaty.
The flap over the Reagan comment may be helpful if it reminds Americans of European doubts. The Reagan administration is considered bellicose by some. The 1980 GOP platform called for arms ''superiority'' over the USSR. During the campaign, Reagan told an interviewer, ''The Soviet Union underlies all the unrest that is going on.''