A wide spectrum of military leaders agrees that the most important military lesson of the Vietnam war is this: Don't go to war unless the people back home support it.

"It seems rather obvious that a nation cannot fight a war in cold blood, sending its men and women to distant fields of battle, without arousing the emotions of the people," retired general Bruce Palmer Jr. wrote in "The 25-Year War," a book about Vietnam. "I know of no way to accomplish that result short of a declaration of war by the Congress and national mobilization."

During the long years of fighting by Americans in Vietnam, no president asked Congress to declare war or even activated sizable reserve forces under his executive authority. Although all the services suffered from this lack of national commitment, the Army suffered most of all both structurally and emotionally.

With no reserve specialists to draw upon, the Army used up most of the experienced sergeants it had in uniform. A former Army chief of staff, Gen. E.C. Meyer, said this led to the "virtual destruction" of the noncommissioned-officer corps, which is the glue holding the Army together.

Gen. Creighton W. Abrams came home in 1972, after commanding the Army in the field in Vietnam, dead set against ever again fighting a war one year at a time without access to critically needed reservists. As Army chief of staff, he started restructuring the division of labor between active and reserve forces so it would be virtually impossible for a future president to go to war without activating the reservists.

Because of the Army's high profile in Vietnam, according to Army Secretary John O. Marsh Jr., that service became the repository for "collective national guilt," and lost self-confidence in the process.

"The moral lesson" of Vietnam, said Gen. John A. Wickham Jr., Army chief of staff, "is that, in any future wars, the soldiers must fight with the conviction that the war is important to the nation. It is the responsibility of the leadership to be sure that the soldiers understand the nature of the war they are involved in. And I think that the moral responsibility of the nation is that, once we commit force, we must be prepared to back it up and win as opposed to just sending soldiers into operations for limited goals."

Tactically, military leaders who led small combat units in Vietnam said the war there demonstrated painfully that success depends heavily on setting clear objectives. They said both the overall conflict and their own engagements, in which men fought and died only to give up a hard-won hill, village or delta soon after the battle, lacked clearly defined objectives.

Wickham and other military leaders said their services also learned some positive tactical lessons from Vietnam, such as how to airlift a large number of troops by helicopter and use them effectively with supporting firepower once they hit the ground. Meyer said success with fast manuevers by light forces in Vietnam emboldened U.S. military leaders to built light forces to respond to distant trouble spots.

Vietnam also revolutionized medical care in the field, with helicopters serving as ambulances. The practice of putting a quick patch on a wound and flying the soldier off to a well-equipped hospital in the rear saved thousands of lives.

Strategically, the Vietnam experience demonstrated that transport aircraft could deliver men efficiently but that the bulk of what is needed in the field must still be shipped there by sea.

Instant communications from faraway command posts in Vietnam to Washington proved a mixed blessing. The president himself could pick bombing targets and make other battlefield decisions that undercut the authority of field commanders. Military leaders have said the lesson for any future war, as demonstrated during the invasion of Grenada, is to concentrate authority with the field commander and his deputies.

Finally, one former Vietnam battalion commander, advised: "Remember, we're watchdogs you unchain to eat up the burglar. Don't ask us to be mayors or sociologists worrying about hearts and minds. Let us eat up the burglar our own way and then put us back on the chain."