Of all the oddities of the fight now brewing over "secret" U.S. connivance in military operations against the government of Nicaragua, surely the oddest is the apparent unanimity of congressional disapproval.

In the recent lame-duck session, the House passed Rep. Edward Boland's amendment, which forbids U.S. support of armed efforts "for the purpose of overthrowing the government of Nicaragua or provoking a military exchange between Nicaragua and Honduras." The vote, by roll call, was 411-0!

Why, one might ask, has this overwhelming expression of congressional condemnation not halted the administration's backing of the "secret war" against the Sandinistas in its tracks? Isn't the president operating outside "the confines of the law," as The Washington Post puts it?

Good questions. The trouble is that the questions, as posed, have no useful answers-- not, at least, short of a presidential capitulation to legal "confinement" that would be indistinguishable from impotence. Does anyone seriously expect a president to throw up his hands every time Congress adds a sweeping rider to a continuing budget resolution?

No one doubts that Congress, consistent with its constitutional role, can close its purse against any policy it dislikes, at home or abroad. But instead of doing so forthrightly, it has in recent years preferred to register symbolic disapproval, while averting its eyes from reality.

Congressmen, who are said to have only 10 minutes a day of reading time, do find time to follow the news. It can be no secret even to Congress that for more than a year the Reagan administration has been helping the Nicaraguan "contras," whose purpose is to wobble or topple the Sandinista junta.

At first the declared aim of U.S. policy was to cut off the flow of weapons and supplies from Managua to the insurgents in El Salvador. But since December 1981, at least, the policy has gone well beyond the interdiction of the arms traffic. You could read about it anytime, anywhere.

The Boland amendment notwithstanding, congressmen are as mixed up as the rest of us about the risks and benefits of U.S. involvement. They, and we, are jerked one way by fearful memory of the Vietnam quagmire and the suspicion that these "destabilizing" efforts are rarely successful or, if successful, beneficial in the long run. They, and we, are jerked the other way by the recognition that there is a war on in Central America and that American friends are under siege.

Obviously, the unanimity of congressional opinion implied by a 411-0 vote for the Boland amendment is a facade, conveniently blending formal disapproval with a wise reluctance to handicap effective resistance.

It is another illustration of the troublesome fact that a system designed in 18th-century conditions does not always offer neat answers to the strains and dilemmas of the late 20th. Rather than debate hollow legalisms, we ought to remind ourselves that it has always been the way of active governments, with far- flung interests, to pursue two-track policies.

One of them was, after all, indispensable to our own origins. Had imperial France not blinked at its formal peace with Britain in 1776, Benjamin Franklin could never have procured the French money and arms that kept Washington's army in the field.

Similarly, had Congress rigidly imposed its formal will in 1940-41, FDR's "undeclared" naval war in the North Atlantic could not have sustained Britain's lifeline in the struggle against Hitler.

Circumstances, you protest, alter cases; and indeed one can hear the critics of Reagan's Central American policy scoffing at the suggestion that such precedents are relevant.

But they are. The pattern endures. Those who raise up a howl over the Boland amendment refuse, it seems, to acknowledge the world's complexity.

If the Reagan administration's secret support of the anti-Sandinista effort is foolish or shortsighted--and it may be; there is no way of knowing now--it will have to be controlled by other than pettifogging legalism.

The very unanimity of the Boland amendment--to say nothing of Congress' reluctance to enforce it by stopping appropriations-- makes it look like little but clever lawyering. It emphasizes a continuing congressional failure to find ways of playing a serious role in the shaping of American foreign policy.