President Reagan's announcement that he may send U.S. Marines to Beirut has elicited from Capitol Hill the usual gush of talk about the 1973 War Powers Resolution. That was Congress' ex post facto riposte to Vietnam--a bold assertion of legislative authority to set limits on presidential uses of military force. Admirable legal energies were spent, alas largely misspent, in this early-1970s attempt to survey and fence the murky zone where a president's foreign policy powers potentially clash with those of Congress to declare war and raise armies. But the power to wage war is intrinsically indefinable and indivisible, political and not legal, constitutional and not statutory. The War Powers Resolution was, then, a sort of fool's errand: decorative filigree on the constitutional edifice.

Yet after Reagan's briefing of congressional leaders last week, the surprising line was that he accepts the congressional pretensions implicit in the War Powers Resolution, although he is not sure just which section would apply should he send Marines to the Levant. One section, contemplating combat or potential combat, requires explicit congressional approval of the use of troops, lacking which a president must withdraw them within 60 days. Another, presumably intended to cover "peaceful" deployments, merely requires presidential notification and sets no deadlines.

When you consider the Lebanon assignment under discussion, the absurdity of these distinctions is clear. In so volatile a setting, there is simply no predictably "peaceful" use of soldiers in battle array. The enduring issue, which the resolution alters only in theory, is one of presidential judgment and congressional responsibility. Under classic U.S. constitutional doctrine, never seriously challenged before Vietnam, a president may use the armed forces in support of his foreign policy, whatever the mood or opinion of Congress.

On the last occasion when the use of U.S. forces in Lebanon was bruited, Dwight Eisenhower sought prior congressional approval. Denied that, he sent 14,000 Marines to Lebanon anyway. Eisenhower's action differed little in principle from the famous action of which Theodore Roosevelt boasted: "I took the canal and let Congress debate; and while the debate goes on, so does the canal." Every presidential use of armed forces short of all-out war, from Jefferson's use of the Navy against the Barbary pirates to FDR's adventures in the North Atlantic in 1940-41 to Jimmy Carter's hostage-rescue plan, has been similar.

If Ronald Reagan is correctly reported--if he has conceded that his discretion in Lebanon is conditioned by the War Powers Resolution--that fact is no less than a milestone in U.S. constitutional history. Richard Nixon, after all, vetoed the resolution, although it was overridden; and it is hard to imagine that any president before the curious Carter-Reagan era would not have done likewise, and for good cause. Vietnam, tragic as it was, did not rewrite the Constitution, or make presidents deputies of congressional majorities.

Congress has many constitutionally explicit means (including refusal to pay the bills) of restraining or aborting military escapades it deems unwise. It should use them. The pettifogging legalism of the War Powers Resolution, planting a legislative foot on presidential turf, is, again, so much filigree.