OVER THE WEEKEND the Costa Rican peace plan for Central America, which was chugging along, all but derailed. This is a dismal development that leaves a conflict-torn region with no diplomatic vehicle going and that throws the United States back upon a military option with poor prospects for success.

One word -- what the diplomats call ''simultaneity'' -- sums up the trouble that befell the plan of Costa Rican President Oscar Arias Sanchez. Like its failed predecessors, his initiative called for relief of the military pressure on Nicaragua in exchange for Sandinista progress toward democracy. But President Arias expected the resistance (and its American sponsors) to turn down the war at once, while the regime in Managua would not be expected to make political concessions until later. No, do these necessary things simultaneously, said -- in their separate accents -- the Nicaraguan opposition, the elected government of El Salvador and the United States. They also asked whether the verification contemplated in the Arias plan would suffice.

To Costa Rica and Guatemala, the starkest danger is continuance of a war that threatens to spill over or otherwise infect them. To El Salvador, the Nicaraguan resistance and the United States, it is consolidation of the Marxist regime in Managua. Splitting that difference has defied the diplomats for most of a decade.

Ronald Reagan stands for giving the Nicaraguan resistance a fair crack at power -- a goal that is sometimes identified, wrongly, with being ''against'' negotiations. But the historical and political realities rendered this goal remote from the start, and Mr. Reagan is increasingly poorly placed to pursue it. Washington's current focus on how the contras were funded during the period of the Boland Amendment is merely the latest grief that his policy has had to bear.

Wary as Washington has been of the Arias plan, then, the dimness of its future is no benefit. It merely underlines a dilemma from which the best feasible exit continues to be a kind of division of labor that puts on Nicaragua's fellow Latins the burden of moving it along a democratic path and that assigns to this country the tasks of preventing Nicaragua from becoming a strategic threat and helping friendly governments in the region cope with the overflow of Sandinista power. That is a policy that most Latins and most Americans could support and which, therefore, Washington could sustain and expect to make work.