Once again, Libya's Col. Muammar Qaddafi has Washington deeply worried. For more than a week now, the actions of this leader of a mostly desert nation of some 3 million people have drawn American newspaper headlines, sometimes eclipsing the news from Central America.
Who is Mr. Qaddafi, and what is he up to?
Qaddafi-watchers are not surprised at Libya's support for the rebel attack on the government forces of Chad, one of the poorest nations in Africa. This support has been well advertised. As an admirer of the late Egyptian President Gamal Abdel Nasser, with a vision of the Islamic state stretching across Saharan Africa, Qaddafi is just doing what comes naturally, the experts say.
Qaddafi has been described by some of his opponents as a madman, or a religious fanatic. But analysts who have dealt with Libya over the years more often than not describe him as a shrewd adversary who is certainly no madman.
And as one expert on Libya, G. Henry M. Schuler at Georgetown University's Center for Strategic and International Studies, has repeatedly pointed out, Qaddafi's brand of militant Islam is not a historical anachronism. Mr. Schuler argues that Qaddafi is in the mainstream of leaders who arise periodically to revive Islamic fervor.
The dream of a united, Islamic Sahara was a goal of Libyan leaders before the turn of the last century. Using Libya as a gateway to the Sahara, the Sanussi family and Sanussiya religious order were the main forces for the spread of Islam through Saharan Africa. The Sanussis became the principal fighting force resisting the French colonialists.
Why should the attack on Chad matter to the United States?
American officials say that impoverished Chad in itself is of little strategic interest to the US. But the fall of Chad to Libyan-backed forces could lead to a new threat against Sudan, a strategically important country bordering Egypt and a key American ally in the Middle East.
Both Chad and Sudan lie athwart the political East-West fault line where Islamicized North Africa meets black Africa. Conflicts across that fault line have at various times sundered the Sudan, shaken Mali and Niger, and affected the most heavily populated African nation, Nigeria. As the Americans see it, Qaddafi is exacerbating splits which are already bad enough.
Colonel Qaddafi does not fit any of the conventional labels, such as ''leftist'' or ''rightist,'' which Western journalists like to apply to the world's leaders. The Soviet Union earns staggering amounts of money by selling arms to Libya. But few scholars seem to think that Qaddafi is a Soviet puppet. What worries US officials is not that Qaddafi might follow Soviet orders, which seems highly unlikely, but that he shares with the Soviets a mutuality of interests in many areas. And as one US official put it recently, ''Libya is, in effect, a very large arms depot filled with Soviet military equipment. At some point, it may be used.''
If the State Department is correct in its reporting, the Libyans are currently using Soviet-supplied equipment in northern Chad, including armor and fighter-bombers.
In addition to the impact Qaddafi can have on black Africa, Sudan, and Egypt, American officials consider Libya's location on the Mediterranean to be important. Libya forms a Mediterranean ''choke point,'' together with Sicily and Tunisia. In a crisis, the Soviet Union might find this strategic location useful.
In an interview with the Armed Forces Journal in late 1981, Egypt's defense minister, Lt. Gen. Abu Ghazzala, said that Libya had lengthened the runway of the airfield at the southern Libyan oasis of Kufra, making it capable of handling Soviet Backfire bombers, which would then be within easy striking range of Chad and the Sudan. According to General Ghazzala, a single Soviet Backfire flew onto the field in mid-1981.
Some experts say they believe that one reason for Qaddafi's attraction to Chad is the possible presence of uranium deposits there. But US officials discount that as a major factor at the moment.
''The man is not crazy,'' said a Libyan in exile who once knew Qaddafi, and who asked not to be identified. ''He is an idealist who thinks that whatever he believes is the ultimate wisdom. . . . He has a tremendous faith in his visions.''
''He's a revolutionary,'' says Mr. Schuler, who has followed developments in Libya for more than 20 years. ''A revolutionary probes with his sword until he encounters steel.''
At this writing, US government officials were hoping that France, which has historical connections with Chad, would provide more of the ''steel'' which they think is needed in that African nation to stop Qaddafi. The United States has airlifted military equipment to Chad and has sent AWACS radar planes to help coordinate air defenses against the Libyans. [Tuesday, French Defense Ministry sources told Reuters that France had ordered a maximum of 200 of its soldiers to the Chadian capital of N'Djamena to serve in a logistical role.]