Les Aspin, 56, whose widely hailed service as chairman of the House Armed Services Committee was followed by 11 tumultuous months as secretary of defense, died May 21 at Georgetown University Hospital after a stroke.

Mr. Aspin, a Democrat and former Army captain, combined credentials as an intellectual on defense issues with the political skill to win election to the House 12 times from Wisconsin.

Although he seemed a natural choice as President Clinton's first defense secretary, everything seemed to go wrong when he took the job. Pressures for his resignation built, and in December 1993, he left office.

Mr. Aspin "rendered our nation extraordinary, selfless service," Clinton said in a statement issued last night. "No one knew better than he how Washington works, but he never thought of it as a game for its own sake. He was here to make a difference. And he did."

Through his quarter-century career in public service, Mr. Aspin was best known for his tenure from 1985 to 1993 as Armed Services Committee chairman. There he brought his intellect to bear on some of the nation's difficult domestic and international problems, all the while showing an encyclopedic grasp of defense and national security issues.

While committee chairman, he became one of the first in Washington to realize as the Soviet Union deteriorated that the Cold War was largely over. Mr. Aspin advocated a reexamination of the entire U.S. military establishment in light of the world changes.

He advocated the "bottom-up" evaluation of the U.S. military force structure when he became Clinton's defense secretary, attempting to pull the Pentagon into a new world that would require it to get the job done with less manpower and smaller budgets.

When he was named defense secretary, many in Washington expected great things. He practically had been in training for the job for 30 years. Yet others warned cautiously that although Mr. Aspin knew the press, the policy analysts and the congressional players, the largest organization he had ever run was the 80-person staff of the Armed Services Committee.

Traits that had helped him stand out on Capitol Hill may have led to problems at the Pentagon and with other Clinton officials. He could awe a listener by ruminating aloud, masterfully examining arcane issues with a kind of brilliant dispassion.

But some in uniform told reporters they would have greater admiration for a secretary who could lead clearly while running meetings that began and ended on time and reached concrete conclusions.

Mr. Aspin held office during a time of turmoil in the world. Almost from the beginning of his tenure, he seemed to take some unfortunate missteps. He publicly undercut Clinton's pledge to lift the ban on gays in the military by saying that Congress and the armed forces themselves would resist the change. He later misspoke about Bosnian policy.

His troubles mounted in October 1993 when, after his denial of tanks and armored vehicles to U.S. forces in Somalia, 18 U.S. troops were killed in Mogadishu after they were pinned down by gunfire. Later that month, the U.S. warship Harland County was turned back from Port-au-Prince, Haiti, by a jeering mob. In November, Mr. Aspin wrote to Clinton that the Pentagon was woefully underfunded in new budget plans and needed an additional $50 billion to carry out the president's own five-year defense plan.

In December, Clinton asked for his resignation as defense secretary.

Many politicians and pundits agreed that the president's entire foreign policy and national security team seemed to be foundering and that Mr. Aspin, who aides said had little personal rapport with the president, was chosen to pay the price alone.

Les Aspin was born in Milwaukee, his father an accountant and his mother a legal secretary. The family settled in an area known for its excellent schools, and Mr. Aspin excelled. In 1960, he graduated summa cum laude from Yale University, where he was elected to Phi Beta Kappa. He then received a master's degree in economics from Oxford University and an economics doctorate from MIT.

From 1966 to 1968, he served in the Army. It was during that time that he worked in Pentagon systems analysis, a discipline that seeks to find solutions to military and national security problems through the use of logic and mathematical and computer models. It was while working as a "numbers cruncher" for then-Defense Secretary Robert S. McNamara that Mr. Aspin turned against the war in Vietnam.

Colleagues from those days recall Mr. Aspin asking skeptical and penetrating questions about the war. Mr. Aspin later told a Washington Post reporter that he became opposed to the war because "the goal wasn't worth the resources that it would require to do it."

He returned to Wisconsin in 1968, first to work in Lyndon B. Johnson's presidential reelection campaign. After Johnson decided not to run, Mr. Aspin made an unsuccessful run for the Democratic nomination for state treasurer.

He taught economics at Marquette University in Milwaukee before winning election from Wisconsin's 1st Congressional District in 1970. The district included the territory along Lake Michigan between Milwaukee and the Illinois line and included such towns as Kenosha, Beloit and Racine.

Mr. Aspin won a seat on Armed Services. Although he had campaigned against the war in his 1970 run and made his first mark on the Hill by churning out witty news releases exposing Pentagon spending blunders, he was hardly "anti-military" and began displaying a remarkable intellect seemingly devoted to Pentagon policy studies.

By 1985, he ranked seventh in seniority among the committee's 29 Democrats. He challenged the octogenarian chairman, Mel Price, of Illinois, for the leadership and won. Suddenly, some people in his own party who had viewed him as a moderately left-wing figure now saw him backing Reagan White House military and foreign policy issues. He supported the MX missile and aid to Nicaraguan contra rebels.

Those stands nearly cost him his chairmanship in 1987. After scrambling to keep his job, he reorganized his staff and began to modify the way he ran the committee, opening up his leadership and inviting other representatives to join his "team."

Perhaps Mr. Aspin's most impressive hour in Congress occurred in January 1991. He seemingly broke with his party again to support President George Bush's hard line against Iraq and predicted that combat could not be avoided. And at a time when uniformed officials were warning of terrible casualties, Mr. Aspin was saying, "I believe the prospects are high for a rapid victory with light to moderate American casualties."

Since August, Mr. Aspin had served as chairman of the President's Foreign Intelligence Advisory Board and was asked to oversee a study of the post-Cold War role of the intelligence community.

His marriage to the former Maureen Shea ended in divorce.

He is survived by a brother, James Aspin of New York.