Libya has not blinked in the face of escalating United States military presence in Chad's civil war. Just as the US was stepping up its military assistance to President Hissein Habre, Libyan leader Muammar Qaddafi put new strength into his backing of rebels in northern Chad.
The result, so far, could be a strategic withdrawal for the US-backed government forces.
The Libyans continued to pound Habre's trapped forces in Faya-Largeau, 500 miles north of the capital here, using Soviet MIG-21 and MIG-23 fighter aircraft and Sukhoi tactical bombers. And it was well-coordinated assaults by Libyan aircraft last week that helped rebel troops recapture the strategic towns of Oum Chalouba and Kalait - both located more than 200 miles south of Faya-Largeau, say Western sources.
In addition, official reports here Sunday said a column of 200 Soviet-built armored vehicles manned by Libyan ''Islamic Legionaires'' and Chadian rebels were only a few miles north of Faya-Largeau, waiting to attack the town.
''With the rebel takeover of Oum Chalouba and Kalait and with the Libyans preparing to fight in Faya-Largeau,'' one US official commented, ''there is no question that the Libyan intervention has reached new heights and the threat to Habre has increased considerably.''
The US announced over the weekend it was sending two AWACS surveillance planes, plus F-15 escort jets and other reconnaissance planes, to monitor what it said was an increasingly grave situation, with reported gains by former Chadian President Goukhouni Woddei's rebel troops. Earlier the US had announced it was more than doubling military aid to Chad, to $25 million.
The Chadian government reported shooting down a Libyan jet over Faya-Largeau Friday, but with a Soviet SAM-7 rocket and not one of the newly-arrived American rockets.
Still, the quiet confidence which accompanied Habre's successful two-week counteroffensive in late July is now replaced here by an atmosphere of tension and despair.
''The military situation is grave,'' one US diplomat said, echoing comments by Chadian and French officials.
Faced with what are reported to be almost continuous Libyan air bombardments and the imminent threat of a Libyan-directed infantry assault, President Habre is now reportedly in the process of deciding whether or not to abandon Faya-Largeau along with the rest of northern Chad and to regroup his troops along a more defendable position stretching from N'Djamena east to Abeche.
This strategic withdrawal, according to Western military analysts, would allow Habre to reorganize his forces and prepare for what many diplomats here consider might be a large Libyan assault.
It was in the context of this deteriorating military situation that President Habre flew from besieged Faya-Largeau to N'Djamena this past weekend. He praised the US for recognition of what he called the significant Libyan and Soviet threat to Chad and the rest of Africa. But he added, ''The aid which the US government has given us is insignificant compared to the massive Libyan attacks and the billions of dollars worth of aid the Soviets are providing the Libyans.''
Libya maintains a 55,000-man regular army, plus navy and air force. Libyan troops fought in Chad on the side of Goukhouni from 1977 to 1979, the year Qaddafi also sent troops to Uganda in an attempt to shore up the regime of Idi Amin. Earlier this year Qaddafi reportedly planned a coup d'etat for the Sudan which was never carried out. But another coup with his blessing, this one successful, took place Friday in Upper Volta.
The US last week sent 30 anti-aircraft missiles and three military advisers to Chad. The advisers were to train Chadians to use the hand-held, heat-seeking missiles.
The State Department, in announcing the sending of the AWACS to monitor air activity over Chad, did not specify where the planes would be based. Spokesmen said, however, that the two AWACS are not the same as the two being used in military exercises with the Egyptian armed forces this week.
President Habre appealed for ''the direct intervention of French aircraft in Chad to stop the Libyan aggression.'' Showing his disappointment at what he considered the paucity of French military assistance, Habre said, ''If France hestitates to aid Chad, it is because there is a strong pro-Libyan lobby in the French government who want the Libyans to commit aggression against Chad without the French intervening.''
But French Defense Minister Charles Hernu Sunday repeated a French refusal to become directly involved in the conflict.
There are, however, indications that the French are stepping up their military assistance to Habre. Numerous French medium-sized transport aircraft have arrived in Chad recently, carrying both anti-aircraft weapons and additional arms.