The Lebanese Forces, Lebanon's largest Christian militia, is rapidly expanding its presence throughout the south of the country, and its leaders are indicating that it no longer intends to disband upon the withdrawal of all foreign forces as it once promised to do.

The force has been growing in size and scope of activity far faster than the national Army, which the United States and France are trying to re-equip and reorganize in a race against time to avert another possible Lebanese civil war.

President Amin Gemayel has pinned his hopes on this new national Army to face down the Lebanese Forces' challenge to his authority and to serve as the backbone of a revived strong central government capable of imposing order and unity on this fragmented land.

Gemayel exercises virtually no control over the Christian militia, and his relationship with militia leaders was described by one western diplomat as "difficult and ambiguous." These leaders include his own father, Pierre Gemayel, the founder of the Phalangist Party, which spearheads the Lebanese Forces and provides 80 percent of its militiamen.

Fears of renewed civil war--spurred in part by the Lebanese Forces' own aggressive expansion--and the faltering performance of the national Army in reasserting itself have convinced the militia leaders that they must be prepared for the worst after Syrian, Israeli and Palestinian forces withdraw.

"We cannot take a chance on disbanding," said the official spokesman of the Lebanese Forces, Fadi Hayek. "If we disband today, the country will go back into anarchy."

After months of internal turmoil following the assassination last September of its iron-fisted leader and the nation's president-elect, Bashir Gemayel, the Lebanese Forces seems to have put itself back together and resumed with renewed fervor its old self-appointed mission as the Christian guardian of Lebanese nationalism and sovereignty.

Its credentials for playing this role may have been seriously questioned both abroad and at home by Lebanese Moslems after its apparent involvement in the massacre of civilian Palestinians in the Beirut refugee camps last September, but Lebanese Forces leaders show no sign of self-doubt or remorse about that episode in their already blood-stained history.

They still heatedly deny that their militiamen played any part in the massacre despite persistent accusations and details on their presence in the camps that have emerged here and in the context of the official inquiry in Israel.

The change in the thinking and objectives of the Lebanese Forces has become increasingly clear over the past few months in its leaders' speeches and its militia activities on the ground, particularly in the mountainous Chouf region southeast of the capital and more recently in the south.

Immediately after Bashir Gemayel's death, the new chief of the Lebanese Forces Command Council, Fadi Frem, repeatedly said he would disband the militia as soon as all foreign forces had left the country. This is no longer the deadline set for going out of business.

Frem, 29, made clear in a recent interview that the Lebanese Forces intends to remain on a military footing long after the final foreign soldier departs from Lebanese soil.

"When the withdrawals take place and the Lebanese Army will be able to fill the vacuum and extend its presence to the entire territory, then the reason behind our creation as a military body will not exist any more," he said.

"Then it will be the president's decision whether he would like to use this military infrastructure we have within the legal infrastructure," he added, referring to suggestions that the Lebanese Forces might be transformed into a national guard, border police or even be integrated into the Army.

Western military experts estimate that, after eight years of sitting in barracks and carefully avoiding combat, the Army will need from two to 10 years before it is back on its feet as a fighting force. They will not even hazard a guess, however, as to when the Army will be in a position to extend its presence "to the entire territory" of Lebanon.

Right now, the Army is only in effective control of predominantly Moslem West Beirut and has yet to take over from the Christian militia in the eastern sector of the capital.

Privately, Lebanese Force leaders are doubtful that the Army will ever become a strong national force capable of reimposing unity on the dozen or so mutually suspicious sectarian communities that make up the Lebanese mosaic.

"We have decided there is no reason to have any illusions or dreams about a strong national Army. The problem is not the Lebanese Forces but the Army," said one Phalangist policy maker.

Frem, in different words, seemed to concur with this assessment. He criticized the Army for the above-average age of its soldiers, its lack of motivation and combativeness, an excess of administrative personnel and the absence of an effective military command center or chain of command.

Frem and other Lebanese Forces leaders have high praise for the new Army commander in chief, Ibrahim Tannous, 53, who once served in the Christian militia and was twice wounded in battles against the Syrians and Palestinian guerrillas.

"If there is anybody in the Lebanese Army to build up a strong, motivated Army it is Ibrahim Tannous," Hayek said. "If he can't do it, I don't think anyone can."

Even Tannous seems to be encountering serious obstacles, however. The government has yet to introduce a national draft to renew the Army's rank and file, while a plan to amend the defense law to allow for reorganization of the top command structure is being blocked by Moslem Cabinet members wary of giving Tannous too much power.

Meanwhile, Lebanese Forces leaders now speak of their militia as constituting "a regular army" capable of playing a "national" role and insist on its right to go anywhere in the country to fill the "vacuum" created by the national Army.

They have been steadily recruiting new soldiers and have increased the size of the standing militia from 3,000 to 6,000 over the past year, partly through call-ups from their 15,000 reservists.

The regular Army has about 17,000 troops, by comparison, but the four battalions the United States is helping to rebuild are still only at 50 percent strength and not regarded as fully operational.

The expansion of the Lebanese Forces has caused nearly a doubling of its annual budget from 400 million to 700 million Lebanese pounds ($108 million to $189 million), which is raised mainly through taxes imposed on goods coming through illegal ports still under the militia's control north of the capital.

These and other taxes recently were doubled throughout the Christian-run enclave covering about one-fifth of the country.

Local press reports say the Lebanese Forces recently purchased $15 million worth of captured Palestinian arms from Israel. It now has 50 to 60 tanks, including Super Shermans and 10 formerly Syrian Soviet-built T-54s, 130 mm and 155 mm long-range artillery, big caliber howitzers and scores of armored personnel carriers, making it a match for the regular Army in armor and fire power, according to Western military analysts.

The bulk of the Lebanese Forces' new recruits are from the Christians living in the Chouf or villages in the south, particularly from among the civil war refugees now returning to their homes. There are conflicting reports as to just how voluntary the service really is.

The rationale for the expansion of the Lebanese Forces into these two areas is linked closely to the return of Christians, who, its political and military leaders argue, need a military presence to ensure their protection from presumed local hostile elements who drove them out during the past eight years of political turmoil in Lebanon.

"It was necessary to have Christians go back to their homes with their heads held up high," said Father Boulos Naaman, superior general of the Maronite order of monks and one of the driving spiritual leaders of the Phalangists.

"We Christians were in a majority there in the Chouf," he said in an interview. "But in the past eight years, the Christians were isolated and besieged by Moslem Druze socialists and communists."

This determination to establish, or reestablish, a military presence in all villages where Christians once lived has led in the Chouf to a bloody feud with followers of Walid Jumblatt, leader of the Druze-dominated and Syrian-backed Socialist Progressive Party, which is leading the resistance to the Lebanese Forces' entry into the mountainous heartland of Lebanon.

The struggle has become increasingly bitter, with ever strident sectarian overtones, as the Druze community has rallied behind Jumblatt, enabling him to reassert his national leadership. This is what the Lebanese Forces, with full Israeli backing, were hoping to prevent.

Frem also points to what he says is the negative impact the thrust into the Chouf has begun to have on the American perception of Lebanon.

"This is precisely the communist plan, that the United States will start decreasing its interest in Lebanon and the communists themselves will be able to regain what they have lost," remarked Frem. "If this continues, maybe they will succeed in making the U.S. initiative in Lebanon fail."

Realizing the ugly turn the Chouf struggle has taken, the Lebanese Forces leadership says it is now trying to deescalate the fighting and strike a political compromise with Jumblatt. While contacts reportedly are taking place, there is no sign of any breakthrough, and Jumblatt is insisting that the Lebanese Forces' barracks, roadblocks and militiamen must leave the Chouf.

Apparently to avoid a similar blowup between rival Christian militia groups in the south, the Lebanese Forces is now seeking to reach an agreement with the Israeli-backed renegade Lebanese Army leader there, ex-major Saad Haddad, over the deployment of its militia in territory he regards as under his sole jurisdiction.

The Lebanese Forces already established a barracks and two training camps in Jezzin in south-central Lebanon last summer. It has recently set up another barracks near Nabatiyeh farther south, adjacent to Haddad's "Free Lebanon" area, and is also now sending patrols down the coastal road as far as Tyre.

Haddad, apparently with Israeli blessings, has unilaterally proclaimed his militia army intends to control all territory south of the Awali River, which is north of Sidon, and is now busy recruiting 3,000 to 5,000 new militiamen.

The Lebanese Forces leaders do not accept his authority, however, and see a "vacuum" developing once the Israelis pull out. They want to extend their armed umbrella over the Christians throughout the south.

Some Lebanese Forces leaders already are talking as if the south is under their jurisdiction.

So far, there has been no major clash between Haddad's army and the Lebanese Forces. But it could occur once the Israelis withdraw, given the past history of intra-Christian rivalries over territory and internecine slayings.

The Lebanese Forces also has avoided any public confrontation with President Gemayel. But this appears to be mostly a result of his giving in to its pressure, or "lobbying," as one official called it, on a number of key issues, including allowing the Lebanese Forces militia to remain intact when other Moslem militia groups were forced in disband last fall.

It is still far from clear how Gemayel intends to deal with the Lebanese Forces over the long run. Although a top leader of the Phalange, Gemayel previously had little influence over the Lebanese Forces, which was commanded solely by his dominating brother, Bashir.

So far, Amin Gemayel has appointed only two former close collaborators of his brother to key posts--Tannous as the commander of the regular Army and Zahi Boustani as chief of general security.

Insiders note, however, that Tannous already was close to the president because the two served together in the Lebanese Forces militia in the Metn area northeast of the capital. Boustani was one of the first of Bashir's aides to come out in favor of Amin when the Lebanese Forces was still badly divided over whom to support as its presidential candidate after Bashir's assassination.

Amin Gemayel slowly is assembling his own team in the presidency, picking almost all his aides from outside the Lebanese Forces inner circles. He has just replaced one former close adviser to Bashir, George Freiha, with Wadieh Haddad, a World Bank economist, as his national security adviser and Cabinet director.

Outside observers note the ambiguous role now being played by Gemayel's father, Pierre, who has reasserted his authority over the party and Lebanese Forces since Bashir's assassination. To all appearances, he has not yet given his full support to his son and has sided more with the militia.

In an interview, the founder of the Phalange, now 77, hinted at his reasons for withholding total support from his son--namely, his doubts about Lebanon ever regaining a strong central authority.

"I hope there will be a time when the state will become a real state, the Army a real army and the police a real police," he remarked, sitting in the same headquarters building where he began 46 years ago. "Unfortunately, we have not reached that point . . . . God alone knows how long it will take."