IT'S IRRITATING to find that Libya has sneaked around the American embargo and bought a transport plane. But that's the least of Col. Muammar Qaddafi's offenses. In 11 years of power he has harnessed Libya's oil billions to his peculiar Islamic precepts and fomented coups, revolutions, separatist movements and terrorism in dozens of countries around the world. His latest move is to invade neighboring Chad, whose uranium-rich northern tier he annexed five years ago. Thus has he carried off the first victory of his Soviet-supplied arms and put himself in a position to extend his troublemaking in Africa.

Ideally, France, still the leading outside patron of formerly French West Africa, would have challenged Libya's "Islamic legions" in Chad. But France had harkened to an African call to permit an "african solution" to Chad's long, ragged civil war, and had withdrawn its troops last year. The legal government then invited Col. Qaddafi in.There was also the matter of a new contract signed with Libya by the French state oil company. Since then, the French have condemned Libya, bolstered their forces in the region and suspended the new contract. African states have demanded Libya's withdrawal and started to address the possibility that Libya will manipulate their Arab/Moslem elements, against their black/Christian/animist elements, the way Libya has done in Chad. But Libya's menace remains real.

The scope of Col. Qaddafi's outlawry is matched only by the scope of other nations' tolerance of it. More than the weight of oil and Soviet patronage is involved. Col. Qaddafi has cleverly exploited his place in Arab, Islamic, "anti-imperialist" and anti-Zionist ranks. So bold and unlikely is his policy -- like Hitler's -- that people have trouble grasping the whole and the purpose of it.

The immediate issue is Chad. The predominantly black and Christian population in the south is not under Libya's thumb and deserves to be sustained by traditional friends in the region, while a close eye is kept on the more fluid politics of the Libyan-controlled Moslem north. African efforts to sponsor elections offering citizens of Chad the choice of living under Libya or not should be encouraged.

In the Carter years the impression spread -- certainly it did in Egypt, a bitter rival of Col. Qaddafi's -- that for all of its depredations Libya enjoyed a certain official indulgence in the United States. The Reagan administration, determined to be tougher on international terrorism and on states that act as Soviet pawns, appears ready to remove that impression. But what judgment must Col. Qaddafi make about American seriousness as he notes that through the 1970s, while virtually all the American allies reduced and in some cases (France) terminated their imports of Libyan oil, the United States tripled its? The largest single part, about a third, of the $20 billion-plus that he acquired last year to finance his dreams of Islamic glory, and his acts of aggression and terror, came from the United States.