Defense Secretary Caspar W. Weinberger warned yesterday against trying to apply limited military power to achieve fuzzy objectives and laid down for the first time six conditions that should be met before Americans are sent into battle.

"Commitment of U.S. forces to combat must be a last resort to be used only when other means have failed or have no prospect of succeeding," Weinberger said. It was the sixth of his conditions in a look-before-we-leap speech that his aides contrasted with recent statements by Secretary of State George P. Shultz.

Shultz told the Trilateral Commission on April 3 that military muscle, not diplomatic effort, was the missing ingredient in pursuing American objectives in Lebanon. He also has called for considering preemptive and retaliatory strikes against state-sponsored terrorism.

The two speeches are almost a reversal of the classic roles of the two secretaries, with the diplomatic chief calling for more military action and the defense chief speaking out for diplomacy.

Weinberger emphatically joined the "never again" school regarding Vietnam in his speech before the National Press Club, declaring: "When we commit our troops to combat, we must do so with the sole object of winning."

Focusing on Central America, he said the first responses to deeper Soviet and Soviet-proxy penetration into this hemisphere should be more economic and military assistance and training, not a few battalions of American troops.

"The president," Weinberger said in his speech, which was cleared by the Reagan administration, "will not allow our military forces to creep, or be drawn gradually, into a combat role in Central America or any other place in the world."

He listed these five other "major tests to be applied when we are weighing the use of U.S. combat forces abroad":

*" The United States should not commit forces to combat overseas unless the particular engagement or the occasion is deemed vital to our national interest or that of our allies. That emphatically does not mean that we should declare beforehand, as we did with Korea in 1950, that a particular area lies outside our strategic perimeter." "

* If we decide it is necessary to put combat troops into a given situation, then we should do so wholeheartedly and with the clear intention of winning. If we are unwilling to commit the forces or the resources necessary to achieve our objectives, we should not commit them at all. If the particular situation requires only limited force to win our objectives, then we should not hesitate to commit forces sized accordingly."

He said Hitler might have been stopped and World War II averted if other nations had sent in a few forces in time to stop him from remilitarizing the Rhineland.

* "If we do decide to commit forces to combat overseas, we should have clearly defined political and military objectives. And we should know precisely how our forces can accomplish those clearly defined objectives. And we should have, and send, the forces needed to do just that . . . ." "The relationship between our objectives and the forces we have committed -- their size, composition and disposition -- must be continually reassessed and adjusted if necessary . . . . We must continuously keep as a beacon light before us the basic questions: 'Is this conflict in our national interest? Does our national interest require us to fight, to use force of arms?' If the answers are yes, then we must win. If the answers are no, then we should not be in combat."

* "Before the U.S. commits combat forces abroad, there must be some reasonable assurance we will have the support of the American people and their representatives in Congress . . . . We cannot fight a battle with the Congress at home while asking our troops to win abroad or, as in the case of Vietnam, in effect asking our troops not to win, but just to be there . . . ." Weinberger, often portrayed as a hawk, told his National Press Club audience that he was phrasing the tests negatively "to sound a note of caution -- caution that we must observe prior to commiting forces to combat overseas."