In its almost obsessive urge to give Libya's Col. Muammar Qaddafi a bloody nose, the Reagan administration has intervened directly-- with military equipment and advisers--in Chad's 18-year-old civil war on the basis of at least three seriously flawed assumptions.

The administration has overestimated Libyan capabilities--as opposed to ambitions --in central Africa; it has underestimated French reluctance to assume the role of anti- Libyan policeman; and it has presumed to take sides in a war about which astonishingly little is known in Washington.

The result is that the United States has become dangerously exposed as the lead country on one side of the civil war; its somewhat unseemly surrogate in the fighting is President Mobutu's Zaire; and whatever the outcome of this round of fighting and foreign intervention, the already fragile internal political and economic situation of Chad will be aggravated and will simply perpetuate the civil war.

The administration claims that Qaddafi wants to control Chad to use it as a springboard for the destabilization of other countries in the region. That may be so, for argument's sake, but can Qaddafi actually do it? And, anyway, what does Qaddafi really want to achieve in Chad? Libya has annexed a strip of northern Chad called the Acuzou strip; it is of little known value, but the Libyans grip tenaciously. What the Libyans want is a government in Chad compliant enough to formally hand over that strip of territory.

To prop up any Chadian government--as opposed to simple destabilization--is an incredibly costly business. The last time Libya was in Chad, in 1981, Qaddafi's promised generosity to what was then the government of Goukouni Oueddi and is now the rebel force never materialized. Civil service salaries were unpaid; basic services, from water supply to the telephone, could scarcely be guaranteed.

Finally, an overtly Islamic, Arab control of Chad would be unacceptable to many of the people in the south of Chad who are black and of Christian or animist faith. Qaddafi would have to be the generous broker of a unified national government--a skill in which he has scarcely excelled in his various foreign imbroglios.

His short-term calculation, which seems to be correct so far, is that foreign support for President Hissene Habre will not be overwhelming and that the rebel forces led by Goukouni Oueddi can gain sufficient ground to force Habre to the negotiating table. This has been a classic pattern of the Chadian civil war in the last six years.

Qaddafi has upped the ante by sending aircraft to bomb the northern stronghold of Faya Largeau. Washington expected France to match Libya, but, to many people's shock, Paris has not done so. This should not, in fact, be a major surprise, though it has considerable implications for some of the U.S. strategic assumptions in sub-Saharan Africa.

France, essentially, is refusing to be the American surrogate in this conflict. President Francois Mitterrand's distaste for military intervention in Africa is well known--he likened his predecessor's African interventions to the behavior of a "pyromaniac fireman." He faces serious domestic economic problems; he has to bear in mind the anti-interventionist feeling among his ruling majority of the Socialist and Communist parties.

With the Socialist Party Congress coming up in the fall, he would have to answer to his own party, which has sought to change France's so- called "neo-colonial" policies in Africa.

Finally, the French are extremely wary of Chad. Historically, the complexity of that civil war has burned the fingers of every foreign power that has intervened. And the French have the longest experience there.

Chad is a country where no government since independence has ever controlled the entire territory. A southern-dominated, pro-French government asked the French to administer northern Chad for the first five years of independence. In 1965, the first revolt, against the fiscal exactions of southerners, took place in the East. Three years later, the north erupted.

It is a measure of the weakness of successive Chadian governments until 1978 that a poorly coordinated revolt, but with genuine nationalist aspirations, was able to keep French-supported governments at bay and finally to topple them. But the rebels were fragmented along regional and personal lines.

The current fighting has pitted two northern clan leaders--both former leaders of the nationalist FROLINAT movement--against each other. Ideology has little to do with their confrontation.

In the meantime, the fighting and lack of administration have reduced this country to tatters. A sense of identity with a central government no longer exists. Southerners, who provide the majority of civil servants and much of the national revenue from the cotton crop, are wary. It amounts to a state of de facto secession. Neither Hissen Habre, whom the United States is supporting, nor the former president, Goukouni Oueddi, has ever been able to establish unified control of Chad.

Under these circumstances, U.S.--or Libyan --military intervention is like applying dirty bandages on a festering wound. Chad will always be vulnerable to Libyan interference unless the United States takes a disinterested lead in ensuring a measure of political justice and economic equity in that country. That is the most effective weapon against foreign intervention--and it applies to more countries than Chad which this administration has identified as "friends" in Africa.