Washington — The use, or threat, of military force as a diplomatic tool has never been a comfortable notion for those in uniform. Military leaders would much rather have a clear-cut mission, be given the means to accomplish it, and the freedom from civilian interference once the political decision to fight has been made.
That same warning about the limits of military power was forcefully expressed Wednesday by Defense Secretary Caspar Weinberger, a man often described as a ''hard-liner'' within the Reagan administration.
Mr. Weinberger in no sense backed away from what he believes are worldwide US treaty obligations and vital national interests. Nor did he soften his warnings about Soviet might and expansionism.
But the most senior of Western alliance defense chiefs did spell out what he sees as important conditions to sending American troops into combat that, in effect, could constrain such deployment.
''National unity of purpose,'' he said, is just as important to the effectiveness of US power as are ''strong and ready and modern'' military forces.
The ''single most critical element of a successful democracy,'' Weinberger said, is ''a strong consensus of support and agreement for our basic purposes.'' Without this intangible thing he likened to a ''moral force,'' he warned, the US could be sapped by another Vietnam.
In essence, Secretary Weinberger was reflecting the concerns he has been hearing from top military leaders in recent years.
It was the Joint Chiefs of Staff, and the civilian leadership at the Pentagon , that cautioned against the stationing of US Marines in Lebanon as a ''peace keeping'' force. Those same groups then argued for troop withdrawal after the disastrous suicide truck-bomb attack took so many American lives. On the other hand (and for essentially the same reasons), the armed services welcomed the Grenada invasion.
In recent months, rhetoric, which some have judged to be more hawkish, has been coming from the State Department. In a speech earlier this year, Secretary of State Shultz criticized ''those who shrink from engagement'' and said ''power and diplomacy . . . must go together.''
Mr. Shultz was speaking specifically of countering terrorism, where he said the United States ''must be engaged.'' He acknowledged that this presented difficult ''practical and moral issues,'' but he has continued to urge more direct US counterterrorist action including military retaliation.
In the speech, Weinberger also acknowledged a new era in which ''the dividing lines between peace and war are less clearly drawn.'' Because of this, he offered ''six major tests to be applied when we are weighing the use of US combat forces abroad.''
He said: US forces should be used as a ''last resort''; they should be used only when the ''vital'' interests of the US or its allies are involved. Once the decision to commit forces has been made, ''we should do so wholeheartedly and with the clear intention of winning''; political and military objectives should be ''clearly defined''; the size and composition of forces in combat should be ''continually reassessed and adjusted if necessary''; and there must be a ''reasonable assurance'' of public and congressional support.
With this nation's 2 million uniformed men and women in mind, Weinberger said , ''the point we must all keep uppermost in our minds is that if we ever decide to commit forces to combat, we must support those forces to the fullest extent of our national will for as long as it takes to win.''