A new group of armed men is beginning to appear more and more prominently alongside the foreign armies and private militias on Lebanon's confrontation lines. It is the Army of Labanon, perhaps the weakest and least motivated of all the armed organizations on the Lebanese battleground, but showing signs of life at last.

Its presence in the field is still minimal. A few hundred outgunned men are cautiously trying to establish their authority in the ravaged suburbs of Beirut and a 500-man unit was deployed with U.N. troops in the buffer zone of the far south last month.

In Lebanon's law-of-the-gun atmosphere-with Israelis bombing Palestinian camps, the Syrian Army skirmishing with Christian militias and the U.N. force confronting heavily armed Lebanese renegade troops-that isn't much. But it is more than the Army has amounted to since the 1975-76 civil war and those modest deployments represent virtually the only sign of progress toward ending the country's long agony.

The Lebanese Army, never very potent, disintegrated during the civil war. Some troops hid in their barracks, some joined one faction or another, and many simply went home. Top-heavy with Christian officers distinguished chiefly by indolence, it was mistrusted by the Moslems who viewed it as a tool of the Maronite leadership.

Now, according to Maj. Mahmoud Mattar, the Army's information chief, the paper strength of the Lebanese Army has been rebuilt to the prewar level of about 18,000.

Mattar said nearly 5,000 of the prewar members have been weeded out or forced to retire, replaced "a new breed" with a "sense of national duty."

He acknowledged that the Army's high salaries are an inducement. With raw recruits drawing a base pay of about $200 a month, conscription has not been necessary, but that also means the government has not had to face a test of whether it is strong enough to enforce conscription.

Officially, all factions in the zany Lebanese political patchwork agree that the Army is the only legitimate armed force and that all should co-operate so it can be rebuilt and restore law and order.

In fact, however, the armed elements that dominate the country-Syrians, Israelis, private militias and Palestinians hold the Army in contempt, and none is prepared to rely on the Army to protect its interests.

As a result, the Army is struggling against overwhelming odds to regain a shred of respectability, while the killing continues.

The top command has been reorganized under a new law aimed at making the Army, traditionally dominated by Christians, politically more acceptable to all sides. With recruits being trained and modest amounts of equipment coming in, mostly from the United States the Army is less of a national embarrassment than its was.

The deployment to the south, even though it drew shelling that apparently provoked some desertions and exposed the Army's overall weakness, was still welcomed as the first time in years that a Lebanese force of mixed religion and background had met any test in the national interest.

Here at the Defense Ministry on the main road from Beirut to Damascus, snappy-looking recruits are marching around and pitching tents and brisk NCOs are rushing around in new Land Rovers.

The Army, however, is near ready to take on the real tasks confronting it-restore order in an anarchic country, challenge the private militias, supplant the Syrian Army as the source of authority in Lebanon and, most difficult of all, serve as the catalyst for a nationwide political reconciliation.

"We know we can't restore order by force," said Mattar. "That would take 50,000 men. We can restore order only if there is a national accord."

In what newspapers accurately called a classic chicken-and-egg situation, the development of that accord, if attainable at all, depends on the reorganization and development of the Army.

Those are taking place, but very slowly and not without challenge.

The impotent government of President Elias Sarkis hopes that reorganization of the Army command, with Moslems and Christians in agreement, will be followed by the resignation of the Cabinet of technocrats that has been trying to run the country since the end of the civil war in 1976 and the installation of a consensus Cabinet of political figures.

They would then work on a national political agreement that would end the divisions that have ruined the nation.

Nobody has any illusions about how remote a prospect that is. Political consensus has not been attained in the Army itself, let alone in the country at large.

For example, under the new law, the posts of defense minister and Army commander must be held by different people. Lt. Gen. Victor Khoury chose the defense portfolio, leaving the commander's post open.

His logical successor, according to military sources, is a respected brigadier, Munir Tarabey. But the same unwritten law that says the president of Lebanon must always be a Maronite Christian applies to the command of the Army, and Tarabey, as a Druze, was unacceptable to the Christian leadership, so he is acting commander while the search goes on.

Military sources in Beirut say the Army is now about 51 percent Moslem and 49 percent Christian, and the percentage of Christians in the officer corps has been reduced from 63 to 54. Many of the new junior officers are Moslems.

That and a new law limiting the formerly absolute authority of the commander are said to have increased Moslem support for the Army. But Western analysts say the number of effective troops ready for duty is still no more than 6,000 because many units are so dominated by one religion or the other that they cannot be deployed without causing more tension than they relieve.

"It's better than it was," one expert said, "but they have a long way to go. The fabric of the Army is so delicate it could be destroyed overnight and the whole place could blow up any second."