NOT SINCE American Marines went ashore at Beirut in 1958 has the United States made a more sweeping initiative in respect to Lebanon than President Carter did in his call for a general conference. Twenty years ago the United States had both the power and the impulse strife, aggravated by foreign intervention, then as now plaguing Lebanon. Today this country has neither the power nor the impulse, and it can intervene only by diplomatic means. But the need, measured in human suffering and the potential for international conflict, is there. The latest Syrian barrages against Christian positions in Beirut underline the point.

For its first 20 months, the Carter administration took a relatively restrained stance on Lebanon, concentrating on trying to keep the lid on and start building up the capacity of the painfully weak Lebanese government to establish order in its own house. Now the president obviously feels that the momentum built up at Camp David and the stature he acquired there can be put put to further use. It is, in our view, a risk worth taking, and it is likely to be popular at home. There is growing sympathy in this country for Lebanon's plight, in particular for the embattled (and pugnacious) Christian minority.

Mr. Carter has in mind a comprehensive approach. He would have a conference with three or four layers of participants: 1) "those who live there," a formulation that would include the 60,000 Palestinians whose presence many Christian regard as the problem; 2) Syria, ostensibly the peacekeeper in Lebanon's Moslem-v. -Christian, left-v. -rightcivil war, and Israel, which fears Syria wishes to swallow Lebanon or otherwise turn it into an actively hostile state; 3) interested foreign countries like the United States and France; and 4) the United Nations, which currently runs a peace-keeping force in the country. The conference would go beyond the establishment of a stable cease-fire. It would also seek a new formula for power-sharing - not partition - between Christians and Moslems. The last formula, set in 1943 when France gave Lebanon independence, is in ruins.

There is much to debate in the Carter approach and, given harsh continuing Syrian encroachments on Lebanon's Christians, not much time in which to conduct the debate. In particular, Syria, which has so far spurned the Camp David initiatives, is likely to look with a beady eye on any American-sponsored undertaking in Lebanon. Such is otherwise the common despair, however, that the president's new plan is likely to be greeted with relief, bordering on thanksgiving in some quarters. Lebanon was once an oasis of democracy, religious coexistence and civility in the Arab world. It is a country crying to be reborn.