Former secretary of state Dean Acheson, a wise man whose many virtues include both a sense of humor and a firm grasp on reality, used to say that he had three boxes on his State Department Desk -- IN, OUT and TOO HARD.

But that, obviously, is not the style of his latest sucessor, Alexander Haig. Having declared Cold War on the Soviet Union, and plunged into an apparently losing power struggle with his own administration, he has now gone into combat against Congress with sweeping demands for the removal of a mixed bag of legislative restraints on the powers of the executive branch to conduct foreign policy.

Among other things, Haig wants more flexibility to dish out military aid and dispatch military advisers to counter Soviet terrorism, subversion and infiltration around the world. He seeks some $350 million in walking-around money for emergency use to get weapons quickly into the hands of forces favorable to the United States. He would repeal provisions restricting U.S. shipments of nuclear material to friends and allies.

Above all, he would erase from the books the so-called Clark Amendment, which has the effect of banning all kinds of American aid, overt or covert, to any of the contending factions in Angola. Enacted in 1976 in the emotional aftermath of Vietnam, its single-minded purpose was to bar clandestine support by the Central Intelligence Agency for insurgent forces fighting to overthrow an Angolan government openly supported by the Soviet Union with proxy Cuban troops.

Leaving the rest of the Haig demands aside as arguable, the issue of the Clark Amendment is one that Secretary Haig might have been far wiser to file away, for right now, under TOO HARD.

A good case can be made -- and was in fact made by the new Republican chairman of the Senate Foreign Relations Committee, Charles Percy, the other day in an interview on Cable News Network -- that the Clark Amendment should never have been passed. Conceding that he had voted for it, Percy now is prepared to say he probably shouldn't have.

One reason he offered is that the Clark Amendment ties the hands of the executive in just one country, when there now exist adequate safeguards in the form of congressional right of review and oversight of all covert CIA activities, of all sorts, around the world. Not everybody, of course, would agree on that.

But that is just the point. By singling out Angola, and addressing a state of affairs existing five years ago, the Clark Amendment does nothing to resolve the really big questions involved in covert U.S. efforts to overthrow unfriendly foreign governemnts. It merely forbids the United States from doing so in Angola.

So why not do away with it and move on to the larger issues of whether the United States should be doing this sort of thing at all -- and, if so, under what kind of uniform congressional constraints?

The answer you get from Percy and others who tend to see the issue in realistic terms is that the Clark Amendment is "unfortunately" the law, that "at some point it should be removed" but that the "timing for doing this is not good not."

Why? Because there is no way the Reagan administration can argue for repeal of the Clark Amendment as a "question of principle" (in Haig's words) without conveying the strong impression of an immediate, practical purpose. What the Haig pitch is saying to the front-line black African states is that he wants to be free to intervene covertly in support of the rebel forces now fighting to overturn the established Angolan government.

At least that's how one of the most important black African leaders heard it. President Shehu Shagari of Nigeria, which is the second biggest source of foreign oil for the United States, has already registered a vigorous warning that if the United States "is willing to support rebels in a soverign African nation, it would be extremely dangerous."

Perhaps that's precisely what Haig wants Africans and others to hear: a tough line, against regimes with Soviet/Cuban connections. Or perhaps he really does see the Clark Amendment question as a matter of principle having to do with "an unnecessary restriction" on presidential prerogatives. Or maybe it's a combination of the two.

But there is at least one other critical principle involved, which has nothing to do with presidential power vis-a-vis Congress and everything to do with public policy. How does the Reagan Administration square its loud complaints against Soviet intervention on behalf of "war of liberation" against regimes friendly to the United States with its equally loud assertion of an American right to do the same thing in the case of governments hostile to out interests?

By putting both those fundamental issues of principle up for a congressional debate centering on the Clark Amendment, Haig is inviting a battle with Congress he does not need and from which he had almost nothing to gain. Unless, of course, his purpose does in fact center narrowly on Angola. In that case, one way or the other -- in Africa or in Congress -- he is almost certain to lose.