Even while we are mourning the tragic deaths of more than 200 U.S. Marines, we have to face a number of hard questions, all of which add up to this one: where do we go from here?

America's servicemen weren't the only thing buried in the rubble near the Beirut airport. Also missing, and perhaps mortally wounded, is most of our rationale for being there.

Our original purpose was clear enough--and a good bit cleaner than our purpose for being in Grenada. We came to Lebanon to try to create some measure of peace among a half-dozen warring factions there, with the aim of giving President Amin Gemayel's government a chance to consolidate its power.

Unfortunately, since all the other factions are opposed, on one basis or another, to the government, we have wound up cast as the protector of Gemayel and the enemy of everybody else--a situation hardly conducive to keeping the peace.

It doesn't matter that we don't think of ourselves as the enemy of the Druze, or the Syrians, or the Palestinians, or the various Muslim sects. We are the friend of their enemy, and, for some of them, at least, that amounts to the same thing. A measure of the confusion in which we are cast is the fact that we still have no clear idea which of the various factions, if any, is responsible for that terrible suicide bombing.

In any case, the bombing was a horrendous exclamation mark, punctuating a sentence that was already fairly obvious: short of sending in an army of occupation, we can neither make nor keep the peace in Lebanon.

The presence of American servicemen--even as part of multinational force--is provocative in that part of the world, a fact that led some observers to call for the replacement of the multinational peacekeeping force by a United Nations unit, preferably without personnel from the United States.

Maybe that is still the way to go. But for the time being, the administration seems preoccupied with hanging tough, letting the world know that it cannot be deterred by thugs and terrorists.

The trouble with that is that thugs and terrorists can change the context in which things happen, and in Beirut they have done so. The serious questions now are not whether we will permit terrorists to deter us from our mission but whether we still have an achievable mission in Lebanon, and whether that mission is better served by beefing up our armed forces there or by discreetly withdrawing them.

No one can disagree with President Reagan's stated objective of creating an atmosphere in which Lebanon becomes governable, or with his contention that peace in Lebanon is the key to stability in the region.

But it does make me a little uneasy when he shifts his rationale for our presence from the rather modest purpose of shoring up Gemayel while his army is trained to the one we are hearing more and more these days: the necessity to remain in Lebanon in order to avoid creating a vacuum into which the Soviet Union would move.

If Lebanon is to be the proxy for East-West confrontation, we will need more than a handful of sitting-duck Marines. If the idea is to create a situation of relative peace in Lebanon, our presence may be more hindrance than help.

That isn't running from terrorists. That is facing facts.