With American peacekeeping troops being killed in Lebanon and with the country itself threatening to break apart, the Reagan administration faces a severe crisis. It has several parts.

The president is moving to fulfill what must be the first policy requirement as long as the Marines remain exposed: to ensure that they have the resources and orders to defend themselves adequately. Mr. Reagan seems to be under no illusion that anything faintly resembling a military solution is conceivable. His evident hope is that a show of readiness to employ force as necessary will help calm down the military situation and give diplomacy a bit more time to accomplish the main thrust of American policy, to try to save Lebanon as a unified state.

The administration has come to understand it is not enough just to strengthen the Lebanese central government. That government has to be preserved as the single available instrument of Lebanese unity and sovereignty. But it must also be induced to share political power more fairly with the non- Christian, less privileged communities that have, in Syria, a tough patron close at hand.

The Christian-dominated government of Amin Gemayel, shying from redistributing national political power, insists that the latest violence stems primarily from international--Syrian, Palestinian, Iranian--intervention. On this basis it pleads for stronger American support of its "infant army." The Reagan administration takes stern note of the Syrian hand behind the Moslem and Druze militias currently challenging the Christian militias and the Lebanese army itself. Mr. Reagan himself, however, making a crucial and necessary distinction, calls the battling a "civil war" that it is Beirut's own responsibility to handle, militarily and politically.

Certainly it would have been better to get serious internal talks under way before the Druze had rolled over Christian Phalange fighters, and bloodied the Lebanese army, in the Chouf area just evacuated by Israel. But the premise of the diplomacy being conducted by American and Saudi envoys, among others, is that there is still some time. Israel, too, has a role: discreetly, to keep Syria from directly entering the fray.

Congress has been rightly concerned that 1) American policy be prudent and 2) the War Powers Act be respected. On policy, there is a substantial majority that either supports the president's measured approach or chooses not to take responsibility for hemming him in. That consensus seems to be permitting a mutually satisfactory War Powers formula to be worked out. No president likes Congress inhibit his foreign policy. But if the peacemakers' best efforts fail, Mr. Reagan could yet be glad he had congressional company.

If the worst happens, and Lebanon slides toward dissolution, the United States will need a new policy. That will be the time to reconsider the American role, including the role of the Marines.