THE ADMINISTRATION is right to set itself a tight deadline, the end of the year, for getting all the troops of the PLO, Israel and Syria out of Lebanon. Abused for years, Lebanon deserves no less. The momentum gotten up at such tragic cost last summer should not be dissipated carelessly. But it's not going to be easy.

The difficulty lies in squaring the formal American commitment to the sovereignty and territorial integrity of Lebanon with a visible and regrettable American tendency to go along with foreign occupation and intervention. Not alone, the United States has acquiesced in Syria's occupation of Lebanon's Bekaa Valley for the last eight years, to the point that the Syrians may wonder how serious Washington is about getting them out now. The United States has accepted, moreover, that Israel has a special security interest in southern Lebanon. Do the Israelis wonder, too?

If Washington is serious, then it will find itself in some hard bargaining not just with Israel but also with Syria and, indirectly, the PLO. Israel's special concern is to trade off its Lebanon presence for as thick a residual political tie with a reluctant Lebanon as the traffic will bear. Syria's concern, presumably, is to ensure that its interest in reclaiming the Israeli-annexed Golan Heights is not lost in the shuffle. The PLO's concern is to engage the United States' attention for its West Bank ambitions. The question of withdrawal may seem narrow and military but actually it is broad and political. 2 That all the parties want something from the United States gives the administration its considerable leverage in the Lebanese context. It also puts on the administration the considerable burden of arranging to fill the dangerous vacuum that surely will be left if the foreign troops depart soon.

The Lebanese will fill part of the vacuum -- and here the Americans can quietly help President Amin Gemayel. Another part of it should be filled by United Nations peacekeeping forces, who, being the instrument of the Security Council, are better fit for their difficult duty than the Israelis are given to acknowledge. A third part will have to be filled by the Italian, French and American participants (and perhaps others?) in the multinational force now policing Beirut. France, in particular, should be encouraged to develop a major presence. In Washington, President Reagan must resolve the tug of war between the State Department, which sees the diplomatic value of a further deployment, and the Pentagon, which thinks mainly of the risks.

Lebanon is not the center ring in the Middle East; the Israeli-Palestinian question is. Ultimately, a Lebanese solution depends on a prior Palestinian solution. The other Arab states, traditionally disdainful of Lebanon, have always felt this way. Some things are possible in Lebanon, however, right now, and they must be done.