The widening war in Chad is turning into a delicate diplomatic test for the government of President Francois Mitterrand. Mr. Mitterrand wants to check the adventurism of Libyan ruler Muammar Qaddafi and to safeguard France's extremely close ties with its former colonies in Africa, many of which have called for direct French military intervention. But Mr. Mitterrand is also sensitive to charges of neocolonialism, and he does not relish being dragged back into the quagmire of Chad's nearly 20-year-long off-and-on civil war.
As a result, the French President has adopted a cautious position. After Libyan aircraft bombed the northern Chadian town of Faya-Largeau, he sent the government of Hissein Habre antiaircraft guns to reinforce an earlier airlift of 400 tons of military supplies. But Mitterrand refused Mr. Habre's requests for French troops and kept the four French Jaguar interceptors based in Libre-ville, Gabon, from tangling with Libyan jets.
''We have no intention of sending troops,'' a French Foreign Ministry official repeated. Although he described the Libyan air intervention as ''outrageous,'' he said at present levels it does not represent a serious threat to government forces holding Faya-Largeau.
This could soon change, however. The air attacks, at first thought to be only defensive measures designed to cover a rebel retreat, have continued longer than expected.
Western officials in Chad Sunday reported that Soviet-made tanks, heading south from Libya, were only a few miles north of Faya-Largeau. The Americans fear this movement foreshadows an imminent counterattack by the forces of rebel leader Goukhouni Woddei, perhaps this time alongside Libyan troops.
The French are hoping the Americans are wrong. Foreign Ministry officials refuse to confirm the tank movements. They say privately that they believe American fears about Libyan adventurism are exaggerated.
''The Americans are obsessed with Qaddafi,'' a French Foreign Ministry official said. ''To be sure, Libya is playing a destabilizing role. But let's not forget that Chad's problems are just as much internal.''
The French say they know this from experience. Since Chad became independent in 1964, they have sent troops four times to keep peace in the landlocked and virtually bankrupt state, each time only to find the country's various chieftains at war soon after they left.
It is not surprising, then, that recent headlines in Le Monde refer to Chad as ''a wasps' nest'' and ''the endless safaris.''
Even conservative French politicians have urged caution. Pierre Messmer, defense minister in the 1960s, said the French had been ''victims of a certain intoxication'' in the past, intervening unnecessarily in strictly local African conflicts.
The French are taking the same line about the coup Friday in Upper Volta that brought to power a paratroop captain with close ties to Qaddafi. American diplomats said the coup may have been ''Qaddafi influenced'' and expressed great concern about the development, but French officials emphasized that it was ''an internal affair.''
''We won't interfere in the internal affairs of other countries,'' a Foreign Ministry official said. ''It's not our problem.''
This attitude represents a conscious effort of France's Socialist government to ''decolonialize'' its relations with French Africa, French officials say. While in opposition, Mr. Mitterrand criticized his predecessor, Valery Giscard d'Estaing, for his close relations with corrupt African leaders such as Central Africa's Emperor Bokassa I, who called Giscard ''my relative'' and distributed diamonds to the Giscard family. Mitterrand also pointed with disgust to Giscard's African interventions.
In power, Mitterrand has been forced to soften his human-rights rhetoric. Leaders such as Guinea President Ahmed Sekou Toure, whose reign has been described as brutal by Amnesty International, have received warm embraces at the Elysee Palace.
Mitterrand has renewed defense accords with the former colonies, leaving some 8,000 French troops in Africa as policemen for Mr. Toure and others. But to some extent, Mitterrand altered the role of these troops. Instead of acting unilaterally as in the past, he has pledged to follow the lead of the Organization of African Unity before intervening.
In Chad, this attitude has left France under pressure from its African allies as well as from Washington. French officials confirm that many of the conservative countries in Francophone West Africa feel threatened by Libyan meddling in sub-Saharan Africa and have been pushing Paris to send troops to Chad to stop Qaddafi.
Semiofficial newspapers in the Ivory Coast have been calling for direct intervention, saying ''Africa is looking towards Paris.'' The French press reported that Guinea's Toure telephoned the Elysee Palace to remind a top adviser to Mitterrand, ''France has its responsibilities with regard to its former colonies.''
The United States, too, has been urging the French to take a more active role in Chad. Reports in the US press to this end reached Paris Monday, but the French were already preoccupied with American military aid to Chad - aid they see as potentially establishing a strong American presence in an area traditionally in France's sphere of influence.
After AWACS surveillance planes and American advisers with antiaircraft missiles were dispatched to assist Chad, Le Monde commented, ''One cannot underestimate the importance of these actions by Washington.'' The newspaper continued, ''For the first time in a zone thought to be our own, we are perhaps about to give way to the superpowers.''
But Mitterrand will not easily permit the US to dislodge France in Africa. He has said repeatedly that it is his goal to keep Africa from falling into the clutches of superpower rivalry. He has also said France remains a world power largely because of its continuing sway over much of its former African empire.
So if Qaddafi does introduce his own troops into the Chad conflict - as some reports already claim - almost all observers here expect Mitterrand to respond with force.