Shortly after Israeli troops stormed through Sidon last month, a Western military analyst was shocked to find Lebanese Army soldiers lolling in their barracks when the city's officials urgently needed help caring for the wounded and digging residents out from under the rubble.

"The soldiers had been ordered confined to barracks when manpower was needed for rescue and recuperation work," recalled the officer, who declined to be identified. "It seems that the Lebanese Army has lost its sense of self-worth. The Army really doesn't have a mission in its own country."

The U.S. government is currently considering sending Marines to Beirut to help the Lebanese Army evacuate Palestine Liberation Organization guerrillas encircled by Israeli forces. As talks drag on, however, one key unanswered question is whether the 20,000-man Army could remain intact and serve as a neutral arbiter if it were given such an important task.

The Lebanese Army, with poorer equipment than the Israeli or Syrian occupation forces in the country, is a fragile institution that has not been in a position to assert the central government's authority for years. It has split along Christian-Moslem lines three times since 1976, and Lebanese and Western diplomatic sources say President Elias Sarkis is reluctant to expose it to any confrontation out of fear that it would dissolve again.

"Today, the Lebanese Army is the weakest player among the foreign military forces occupying Lebanon," said a Lebanese Army spokesman who would only identify himself as Commandant Malouf.

Lebanon is currently carved into four military regions, dominated by Israelis, Syrians, Christian militias and U.N. peace-keeping troops. The nation's own Army does not control any of them.

"It is impossible for the Lebanese Army to operate in a situation like this," a Christian militia officer said.

The Army's disintegration began during the height of fighting in Lebanon's 1975-1976 civil war, the second major civil conflict since 1958. The estimated 3 million people of Lebanon include adherents of 17 different Christian and Moslem communities, which have existed side-by-side for centuries with periodic flareups and mutual massacres. All the groups were represented in the Army, but the soldiers broke away from their units to fight alongside their co-religionists.

In rebuilding the Army, Lebanese officials have been careful to keep the rank-and-file soldiers at a 50-50 ratio between Christian and Moslem, the Western military source said.

Part of the difficulty in reconstituting the force lies in the cultural traditions and economic conditions of the disparate groups.

Well-educated Maronite Christians, for example, have traditionally gone into the Army in large numbers, whereas the mercantile Sunni Moslems sneer at the military as a career, preferring to trade, and the Moslem Shiites, the bulk of Lebanon's poor, lack the education to graduate to officer ranks.

As a result, large numbers of Lebanese Moslems continue to see the Army as the tool of the Maronite Christians. They do not want its soldiers posted in their areas, preferring to rely on local militias.

The head of the Army, Maj. Gen. Victor Khoury, and the head of Army intelligence, Col. Johnny Abdo, are both Maronite Christians, but the Western analyst stressed that there were Moslems throughout the command structure.

"Yes, there are Moslems who believe the army is controlled by a Maronite cabal," the source said. "But that is less true today than it ever was in the past."

Critics and supporters of the Lebanese Army said it acquitted itself well in its severest test yet--three months of fighting in Beirut last year against the Palestine Liberation Army. The PLA is equipped and officered by Syria.

"Moslem and Christian soldiers fought side-by-side then," said spokesman Malouf. "The Lebanese soldiers come from these communities that have fought each other so fiercely. The same fighting qualities exist among our soldiers."

Malouf acknowledged that the Army suffers from low morale but argued that the problem will only last until the last foreign soldiers leave Lebanese soil.

At present, the Israeli Army controls half of the country from the west side of Beirut south to Israel and southeast to the Syrian border in the southern Bekaa Valley. The Israelis are aided in areas near their border by the militia of the Christian rightist Saad Haddad. Also in the south, U.N. peace-keeping troops occupy two buffer zones, areas that the Israeli Army overran or bypassed at the start of its invasion of Lebanon June 6.

In the north, the Lebanese forces led by Maronite Bashir Gemayel have carved out a 600-square-mile enclave that runs from Beirut's east side north along the coast for about 40 miles to Barbarah and stretches east almost to the Bekaa Valley town of Zahle. The rest of the country is controlled by Syrian troops, although there are two PLO refugee-military camps outside the cities of Baalbek and Tripoli.

The Israelis, who have invaded PLO bases and Palestinian refugee camps inside Lebanon with impunity since 1969, have the strongest force with seven Army divisions, between 75,000 to 110,000 men, said a Western military analyst. Next are the Syrians with a force of 30,000, mainly in the central and northern Bekaa Valley. With a callup of reserves, the Lebanese Forces could field a force of 20,000 men "very quickly," said the analyst.

Before the Israeli invasion, the PLO forces stood at 20,000 also, but they have been badly mauled by the Israelis and probably total between 8,000 and 10,000, the military source added. About 6,000 are encircled by the Israelis in West Beirut, and small numbers of reinforcements are in Baalbek and Tripoli, he said.

In addition to these major groups, Lebanese Christians and Moslems are divided among themselves by countless splinter groups, which fight each other for political ascendancy. There are also about 80 autonomous armed groups ranging from disciplined private armies to gangs of armed thugs, according to the current survey of the International Institute for Strategic Studies.

Army spokesman Malouf declined to comment on observers' speculation that the Army could fall apart again if the civil war renews. "That is a matter of politics that I would not like to go into," he said.

"If we had an Army with a mission, morale would not be a problem," Malouf insisted. "In the end, we would just like to be able to defend ourselves and restore our sovereignty."