Should the U.S. launch a preemptive military strike against a new plant in Libya that Washington describes as the world's largest chemical weapons factory?

That's what President Reagan is threatening to do. But such a military move would constitute an act of war, a decision that should be left to George Bush and not taken by a chief executive only days away from leaving office.Even if this hot potato is passed to Bush, he would do well to explore all the alternatives - including economic sanctions - before bombing Libya again. So far, Washington has been unable to muster support from its usual allies, including even such a staunch friend as Britain. If the U.S. gets tough with Libya, it had better be prepared for a sharp backlash.

Up to a point, a strong case can be made that Libya is lying when it says it is only building a pharmaceutical plant, not a factory for producing nerve and mustard gas. If it's only a pharmaceutical plant, why is it surrounded by a high wall, reportedly protected by Soviet-built anti-aircraft missiles, and adjacent to a metal fabrication plant that could be used to make canisters and warheads for containing poison gas?

Moreover, the case for getting tough with Libya is certainly strengthened by strongman Moammar Gadhafi's long record of encroaching on his neighbors, exporting terrorism throughout the world, and engaging in deliberate provocations like the one this week in which two Libyan jets approached U.S. forces in international waters in a hostile manner.

But beyond that point, Washington's case for getting tough with Libya starts to break down. Even if Libya is building a chemical weapons plant, it is breaking no law by doing so. Though the use of chemical weapons in international conflicts is outlawed by the 1925 Geneva Convention, there are no international agreements prohibiting their production.

Moreover, Washington weakened its own case by rejecting Libya's offer to permit an international inspection of the new plant.

Washington spurned the offer because such facilities can switch from making poison gas to making pesticides - and vice versa - in only 12 hours. But repeated inspections could disrupt any clandestine weapons production. Moreover, by spurning the offer the U.S. has muffed an opportunity to press for surprise inspections at the Libyan plant.

Maybe the U.S. can still retrieve part of what it has thrown away. Next week the representatives of more than 100 nations will meet in Paris to discuss ways to get tougher on chemical warfare. Libya will be among those in attendance. The U.S. can and should use this conference to put Libya on trial before the court of international opinion.