THE DEATH of a Marine in Beirut is bound to quicken the debate already gathering about the American deployment in Lebanon. The American died, and three others were wounded, not as a result of hostile fire but in a mine-clearing accident. Inevitably, however, the incident will fuel anxieties about the mission as a whole. This is not bad. The mission is a proper one, but it was launched with too little public or congressional discussion. Better to look hard now than to wait for a larger crisis to trigger a review.

The administration did not rush into Lebanon casually -- not in August when a force was sent to cover the exit of the PLO and not now as the force returns to help stabilize Beirut until other foreign armies leave and the Lebanese government feels it can cope on its own. On the contrary, although the State Department was quick to see the value of a steadying American presence, the Pentagon has been notably hesitant to put American troops into a situation where, it feared, casualties might be incurred, the mission might run on and the public might be scared out of supporting the administration's arms buildup.

Mr. Reagan weighed the Pentagon's reservations heavily in dispatching Marines in August, setting a 30-day deadline, for instance, and beating it. This time, having been reminded by the Beirut massacre of the volatility of the scene, he has leaned more toward the State Department view: no deadline. That is the wise course.

The administration insists Marines are being sent in circumstances of cease-fire and with the agreement of the host government and the principal parties: for stabilization, not for "peace-keeping," the latter implying there is a peace to be kept -- hence, danger for the peace-keeper. The distinction is jarring and more than a little arbitrary and political in light of the many possible ways, involving provocation and conspiracy and not just accident, in which Americans might get caught up. It would be foolish to rule out complications far more serious than the mine-clearing accident.

It does not seem to us, however, that the United States is heading into a Vietnam-like "quagmire," in the common phrase. The situation is different, the American role is different, the measure of awareness (and wariness) is different. If the length or cost or point of the Lebanese involvement began to weigh heavily, we have no doubt that it would be promptly reviewed--not necessarily terminated but reviewed to keep the involvement within the bounds of informed public and congressional consent.

"Quagmire" is still for many Americans a controlling metaphor, an automatic reflex against foreign risk-taking. What should be of at least equal concern are the signals of indifference and inconstancy that a preoccupation with that danger sends to friends and adversaries alike. It could take time for Lebanon to settle down; there could be casualties and spells of bad news. But the goal, while it remains in sight, is surely worth pursuit.

Congress has a special interest on account of the War Powers Act. The passions about "presidential war" that produced this legislation are much weaker now. There is a certain view, inside and outside the administration, that the act is an obsolete relic of the guilt-ridden 1970s, a positive hindrance to the executive flexibility required and accepted in the chastened 1980s. But the act remains the law of the land, and Congress has a strong institutional interest in seeing that its terms are honored.

Fortunately, Congress also seems to recognize a strong national interest in a sound policy in Lebanon. There has been a little jousting between Mr. Reagan and various legislators--one of them writes on the opposite page today--over whether "hostilities" are so imminent in Lebanon that the president needs Hill approval to keep the Marines there past 60 or 90 days. This should not be a difficult matter to work out, given the general congressional disposition to support the president in Lebanon and given, too, the administration's wish to ensure that its Lebanon policy enjoys broad public and congressional favor.