Chad's long civil war, intensified by the battlefield array of modern Libyan and French weapons, is likely to continue unabated for years even if Libya and France eventually withdraw without fighting.

The Libyan and French intervention on different sides, with France reluctantly coming in to halt Libya's land grab, places two old actors on the Chadian stage in direct military confrontation here for the first time and increases the chances that Chad's 18-year fratricidal carnage will broaden into a regional conflict. This prospect worries two neighboring American allies, Sudan and Egypt.

Although Chad is of almost no strategic or economic value to any other nation except Libya, its civil war has led to the Libyan bombing of Sudan's western border regions in 1981 and, more recently, erupted in nasty border fighting between Chadian dissidents and the Nigerian military around the islands on Lake Chad from April to June this year.

Libya bombed Sudan while trying to wipe out the tough guerrilla bands of Hissene Habre, Chad's current president.

From his Sudanese refuge, Habre was able to launch an attack that in 1982 toppled the Libyan-backed coalition government of former president Goukouni Oueddei. It is the Goukouni group of factions, united only in their opposition to Habre, that along with an estimated 3,500 Libyan soldiers has taken over the northern third of Chad since June.

French intervention in August halted the mechanized Libyan and rebel advance south of the captured town of Faya Largeau. Libya has fortified the Sahara Desert town with its Soviet-made tanks and armored cars and has moved in the gasoline that will be needed if the rebels try to drive south to the ultimate goal of all the combatants in almost a generation of war, the capital city of Ndjamena.

France has committed more than 3,000 troops to its "Operation Manta," an unknown number of combat aircraft (eight of them based in Ndjamena), surface-to-air antiaircraft missiles for use if Libya chooses to resume its strafing and bombing attacks, armored cars and an undisclosed lineup of sophisticated ground weapons.

Both Libya and France, each keeping its Chadian ally reined in for the moment, have observed a tacit cease-fire since Libya halted its bombing attacks on the government-held northern town of Oum Chalouba on Aug. 12.

Yet during the lull, as in the past, the Chadian antagonists are being equipped by Libya and France with some of the most modern weapons from the eastern and western arms markets and are being retrained and reorganized to go at one another again if France can ever extricate itself from its former colony by securing a Libyan withdrawal.

There are no ideological differences among the multiplying factions and, therefore, there is no basis for a political compromise. The fighting here follows patterns set in the centuries before France conquered the region by beating one group at a time between 1900 and 1914.

The fighting began again five years after independence in 1960. The issue in Chad's war is which of its uncounted ethnic factions will govern the country. The current intervention of first Libya and then France, with American urging, and the previous years of Sudanese-Egyptian manipulation and aid to any faction that claimed it was anti-Libyan have not changed that.

French President Francois Mitterrand's current diplomatic initiative, in talks through his envoys with Libyan leader Col. Muammar Qaddafi's officials in Tripoli and in Addis Ababa with the chairman of the Organization of African Unity, Ethiopian head of state Mengistu Haile Mariam, follows many similar attempts in the past.

Each faction has used such periods of truces and talks to shop for more arms from the weapons stockpiles of Libya, France, Sudan and Egypt.

The OAU has two years-old reconciliation committees on Chad, one "standing" and one "ad hoc," which have achieved little but time for the groups to rearm.

The United States and France, in a successful maneuver to get Qaddafi's 9,000 troops out of Chad at the end of 1981 after their first year-long invasion, financed an all-African, multinational OAU peace-keeping force in Chad to keep the factions from fighting until elections could be held.

But ex-president Goukouni's forces attacked Habre's forces from behind the OAU lines in a fruitless effort to provoke Habre into firing on the peace-keeping force and antagonizing the OAU.

The OAU force of Nigerian, Senegalese and Zairian troops was ineffectual and was withdrawn simultaneously with Habre's capture of Ndjamena from Goukouni in June of last year.

Since then, most of the 50-nation OAU has recognized Habre's faction as the official government of Chad. After winning that recognition, Habre made it clear that his government would not reconsider previously discussed positions on reconciliation.

Libya on one side and France, Sudan and Egypt on the other, reportedly with the encouragement of the United States, have contributed to the breaking of past truces by arming their favorite faction of the moment. Each of the factions, presenting themselves as pro-Libyan or pro-western on different occasions, has used Libya's dubious claims to Chad territory and the West's desire to keep Libya out of Chad to win backing from both.

Since Qaddafi has not wavered in his dream of creating from the region's weaker states a Libyan-led, pan-Islamic revolutionary state stretching from Senegal on Africa's Atlantic seaboard to Sudan on the Red Sea, the Libyan-western competition in this weak and poverty-stricken country is unlikely to end in the foreseeable future. The coming to power of one of Qaddafi's admirers, Capt. Thomas Sankara, in an early August military coup in nearby Upper Volta is likely only to encourage Qaddafi's efforts.

On a personal level, Habre is described by a western diplomat here as the quintessential desert warrior in a land where most male Moslems see themselves in the same light. As Chad's head of state, the diplomat said, Habre is unlikely to go to a "roundtable" negotiation as just another factional leader or to regard the many warlords as his equals.

On its side, the Libyan-supported coalition of rebel forces broadcast over its Radio Bardai Sunday that it is willing to seek a "sincere reconciliation" with Habre on the "precondition" that French troops be withdrawn. In the face of the Libyan presence, acceptance of such a proposal would be suicide for Habre's government.

As has been the case before, both Chadian groups have established conditions that will make a face-to-face meeting difficult if not impossible to achieve. Among Chadians, these preconditions rarely are put forward as just bargaining positions. It is hard to withdraw them for face-saving reasons.

Nigerian President Shehu Shagari, in a speech delivered just before the current round of fighting ended three weeks ago, poignantly described the Chadian dilemma in one sentence. "I do not believe," he said, "that any one leader in Chad is capable of bringing about peace in Chad."