After months of tension with the Pentagon over the conduct of NATO's war against Yugoslavia, Army Gen. Wesley K. Clark was abruptly informed yesterday that his term as the alliance's top commander will end in April.

Senior Pentagon officials, disclosing the move, portrayed it as a normal rotation. But the decision to end Clark's term a few months short of three years was unusual, and some military officials said it may be seen by his congressional supporters and among European allies as an affront to the general who led NATO to victory.

Clark had expected to stay at least a full three years until July 2000 and possibly longer, because most of his predecessors as the supreme alliance commander in Europe served in that post for four years or more. Informed of the decision less than an hour before a reporter called seeking his response, Clark later issued a statement accepting the change and saying he would continue to fulfill his responsibilities at NATO until next spring.

In an effort to make clear that the decision was not retaliation against Clark for his differences with the administration during the war, Pentagon officials said four-star officers in several other regional commands also will step down next year after three-year tours. They noted that Clark was the longest-serving regional commander in the group, having headed U.S. forces in Latin America for one year before taking the NATO post.

They also said the timing of Clark's departure had been dictated by a desire to move Air Force Gen. Joseph W. Ralston, vice chairman of the Joint Chiefs of Staff, into the NATO job. Ralston had been planning to retire to Alaska when his term as the Joint Staff's second-highest ranking officer ends in February.

Defense Secretary William S. Cohen, who values Ralston's skills as a military and diplomatic trouble-shooter, was particularly eager to retain the Air Force general, aides said. The NATO position -- traditionally the most prestigious of the regional commands -- was the only one that could entice Ralston to stay in uniform, officials said. By law, Ralston would have at most 60 days from stepping down as vice chairman to assume another command or else be required to retire.

In a telephone interview late yesterday from Japan, where he is on an official visit, Cohen said he would have preferred to wait until autumn to make a decision but felt obligated to give Clark as much notice as possible. He said he had discussed with the White House the possibility of offering Clark an ambassadorship in the interest of retaining the general's talents as well.

Asked whether he would accept such an offer, Clark, in a brief phone interview from Brussels, said: "I'm not going to speculate on what opportunities might be available in public life or private life. There is a long time yet to do this job, and I've got to keep my attention on it."

"There are going to be a lot of people in the alliance who are going to wonder what has happened and why," said retired Adm. Leighton Smith, formerly a U.S. commander in Bosnia who worked with Clark. "But I don't think his authority will be undermined because his subordinates are professional officers and will respect his orders."

Asked about Clark's departure and the perceptions it might create, the White House declined to comment.

The selection of which U.S. general will lead NATO is arguably the second most significant military appointment that any American president and defense secretary must make, after naming a chairman of the Joint Chiefs of Staff, given not only the tensions in the Balkans but relations with Russia and the European allies. Clark, 54, has brought to the job skills as both an accomplished soldier and a deft diplomat. He was the senior military member on the team, led by diplomat Richard C. Holbrooke, that brokered the 1995 Dayton peace accords that ended the fighting in Bosnia.

While he was Cohen's pick to take over as NATO commander in 1997, there were tensions between the general and the Pentagon leader over how to deal with Yugoslav President Slobodan Milosevic's aggressive moves on Kosovo starting months before NATO's decision to go to war. Clark favored a tougher U.S. and NATO posture toward Milosevic, with the use of force if necessary to get the Yugoslav leader to comply with his agreements to respect the rights of ethnic Albanians in Kosovo.

During NATO's 11-week air campaign last spring, Clark repeatedly pushed for more aircraft and a broader assault. He also urged the Pentagon to allow him to plan a ground invasion of Kosovo if the airstrikes failed to drive Yugoslav forces from the Serbian province. While top administration officials had held to the hope that the air operation would succeed, they eventually authorized some planning for a land invasion.

Cohen insisted that any strains with Clark had played no part in the decision to move Ralston into the NATO job, describing his relationship with the Army general as "good and cordial."

Controversy has followed Clark throughout his career. While he has a reputation for intelligence, and moves comfortably between diplomatic and military worlds, he also has drawn resentment from military colleagues for being "too political" and for micromanaging subordinates.

Like President Clinton, Clark grew up in Arkansas and attended England's Oxford University as a Rhodes scholar. But the two did not know each other as students in Arkansas, and their time at Oxford did not overlap.