French officials are indicating - privately - their dismay at the Lebanese government's rejection of the French truce plan for Lebanon because, the French insist, the idea was proposed to them by Lebanese President Elias Sarkis.

Under the plan several thousand men of the Lebanese Army - half of them Christians and half Moslems - would place themselves between the warring Syrian forces and the Christian militias in Beirut.

The Lebanese government rejected the idea Thursday after Moslem members of the Cabinet opposed it.

The French said they went along with Sarkis' proposal, even though they had doubts about the effectiveness of the Lebanese Army, because they wanted to do all they could to try to establish a lasting cease-fire in an area of longstanding French interest.

Since Sarkis' failure to get his own government's support, suggestions have been circulating in the United Nations that U.N. forces, led by the French regiment already stationed in southern Lebanon to separate Palestinian guerrillas and Christian militiamen there, should be move north too Beirut to separate the Syrians and Lebanese Christians.

French sources indicated, however, that the French government would consider that idea only if a cease-fire were signed. The French also expressed doubt that the Syrians would accept French forces in Beirut or that the Soviet Union would not veto such a proposal in the U.N. Security Council.

The French said they believe the idea is being circulated at the United Nations nevertheless as a way of telling the Syrians that they are going too far in their efforts to crush the Christians in Beirut.

The Sarkis government's rejection of the French-sponsored plan has been a political embarrassment to the government of President Valery Giscard d'Estaing. In the midst of debate in the National Assembly earlier this week on a Socialist no-confidence motion, Socialist leader Francois Mitterrand received prolonged applause when he endorsed Giscard's peace efforts.

Ever since the Crusades, France has been the traditional protector and patron of the Christians of Lebanon. The country became independent during World War II after the League of Nations carved it from the Turkish empire's Syrian province as a French mandate after World War I.

Educated Lebanese Christians still look to France as their cultural motherland and Paris is filling up with wealthy Lebanese Christian refugees.

The French regard the present fighting in Lebanon as the almost inevitable byproduct of the Camp David agreements between Israel and Egypt. Not only was the Israeli-Egyptian accord bound to create a reaction among Arab radicals, French officials have said, but the Syrians and Palestinians were also bound to consider that, at least for a time, Israel would feel it must restrict its military actions so as not to embarrass Egyptian President Anwar Sadat, who must try to persuade Arabs that he has not betrayed the general Arab cause.

Some Frenchmen believe that both the Syrians and hawks among the Lenanese Christians may have sought a confrontation. The Syrians, they say, saw the opportunity to act against the militias while Israel was inhibited, and the Christians saw the opportunity to internationalize the Lebanese conflict, as they have always wanted to do.

In this view, it is possible that the Syrians welcomed the provocations of the Christian militias. Pierre Gemayel and Camille Chamoun, the leaders of the main Christian military organizations, have for a long time wanted U.N. forces to be placed in Beirut to protect the Christian neighborhoods against the Syrians.

The French do not believe, however, that the Syrians seek either to liquidate the Christians of Lebanon or to annex the country, despite the historic Syrian claim that the League of Nations partition was illegal.

Some Frenchmen believe that the Syrians want to terrorize the Christians to induce large number to go abroad and so reduce the still economically an culturally preponderant Christian community of Lebanon to move manageable proportions I.

This is not incompatible with the dreams of many Christians to gather the community into the enclave around Jounieh north of Beirut and to create a small, exclusively Christian Lebanese state. Also pointing in the same direction is the idea President Carter is said to favor of dividing Lebanon into Swiss-style cantons containing segments of the country's different communities.

Some Syrian officials are said to believe that their army has overreacted to the Christian military pinpricks. If the Syrians do not stop now, it is thought here, Western opinion will start mobilizing behind the Christians.

Meanwhile, however, the French seems to think they have made the only diplomatic moves possible. The French government was caught off guard by the Syrian offensive, and the leaders who could make decisions on new moves are accompanying Giscard on a state visit to Brazil.

France can be expected to stay in the diplomatic game, however. The French have long sought to get back into the Middle Eastern picture, from which they were excluded by Israel's contention that France had placed it self outside by championing the Arab cause.

In Lebanon, Israel and France have a mutual interest in trying to ensure the survival of a strong Christian community. The French are also proving useful to Washington as an alternative Western voice with some independent moral authority of its own in Syria and Lebanon, a traditional sphere of French influence.