Chemical Reaction As the U.S. presses Libya over a nerve-gas plant, a shootout erupts. Did Gaddafi sacrifice two planes so Washington would take the heat?

The unlikely combination of Ronald Reagan and Muammar Gaddafi resembles nitroglycerin: it can produce an explosion at the slightest jolt. Last week, for the fourth time since 1981, just such a blowup took place in the Mediterranean skies off Tobruk, where a shootout that could have been taken right from the movie Top Gun ended in the downing of two Libyan jets by American pilots.

This time, however, there was a major difference. While the first three incidents occurred when Washington decided to swat the desert dictator, the latest confrontation was wholly unexpected. When the Libyan MiGs were destroyed after they persistently pursued two Navy F-14 fighters protecting the carrier U.S.S. John F. Kennedy, the U.S. found itself on the defensive not only militarily but also in its international relations.

. The eruption came as the Reagan Administration was applying calculated pressure on Gaddafi, and on U.S. allies, to prevent the production in Libya of poisonous gases that could be used in chemical warfare. The U.S. insists that a huge chemical plant at Rabta, 50 miles southwest of Tripoli and ringed with antiaircraft batteries, is primarily intended to produce mustard gas and chemical nerve agents. In a pre-Christmas TV interview, Reagan refused to rule out the possibility of a military strike against the plant. On background, Pentagon experts even suggested that Tomahawk cruise missiles, which can be launched by surface ships or submarines from as far as 800 miles away, might be used to level the suspect facility.

The clash that followed -- perhaps intended by Gaddafi -- threw the focus back on Washington's seeming eagerness to swing a big stick at easy targets. Soviet Foreign Minister Eduard Shevardnadze noted that the dogfight had "poisoned the atmosphere" as 142 nations opened a five-day conference in Paris over the weekend on ways to stop the increasing spread of chemical weapons. "Gaddafi must be pleased over the incident," said an Italian official last week. "It gives him a chance to play the victim."

It did little good for a presidential spokesman to protest that "we didn't try to pick a fight" or for senior U.S. officials to minimize the possibility that the U.S. would take out the weapons plant by force. Arab states lined up in the United Nations to denounce America's "brutal aggression." In the harshest language the Soviet Union has used toward the U.S. in two years, the Kremlin labeled the American action "state terrorism."

In Western Europe jittery American allies wondered whether Reagan was once again indulging himself by kicking his favorite terrorist -- and what the cost would be. Military bases went on alert in Italy, where Lampedusa Island was the target of an amateurish Libyan missile attack after the U.S. bombing of Tripoli in 1986. Britain supported the U.S. assertion that Rabta is intended for weapons production, but the Thatcher government urged Washington not to attack it. The French, who are host to the chemical-weapons conference at UNESCO headquarters, were irritated. The sharpest criticism came from the leftist Paris daily Liberation: "Gaddafi has lost two planes, but Reagan hasn't necessarily won out. These two were made to detest each other . . . One can understand that their farewells would be agonizing."