Along comes the 200th birthday of the Constitution just when we can use an occasion for fresh judgment of the great and abiding question of constitutional government posed anew by the military actions Ronald Reagan has taken, without formally consulting Congress, in Grenada, Lebanon, Libya and now the Persian Gulf.

And along comes Arthur Schlesinger Jr., historian and Democrat, just when we can use a well-guided tour of the realities as well as the roots of the ultimate question of which branch is constitutionally empowered to commit the country to war.

It is, of course, Congress. Schlesinger drew this familiar tenet this week not merely from Article I of the Constitution but also from the materials bearing on the currently much-discussed ''original intent'' of the Framers. He found the Framers ''unambiguous'' in reserving to Congress the vital powers in international affairs, above all the power to declare war, and he rejected the use modern presidents have made of the commander-in-chief clause as a vehicle of sweeping claims for unilateral executive authority.

Still, he acknowledged in an address to the Center for Strategic and International Studies, ''the text of the Constitution was too full of generality, ambiguity, omission and overlapping grants of authority to settle the range of problems arising in the conduct of foreign affairs. The result, as E. S. Corwin famously put it, was to make of the Constitution 'an invitation to struggle for the privilege of directing American foreign policy.' ''

From early times, Schlesinger reminds, ''unauthorized presidential adventurism'' thrived. Strict constructionist Jefferson secretly sent a squadron to fight the Barbary pirates. Twentieth century presidents went on to put the country into several wars, doing so, moreover, not simply by usurping power, ''which creates no constitutional precedent,'' but by illegitimately expanding the president's constitutional claims.

Like most administrations over the past 40 years, Schlesinger said, this one, starting in Grenada, has trampled on original intent. Apply it to foreign policy, he said, or ''shut up about it altogether.''

Schlesinger would ''shut up about it altogether.'' He believes that the Framers' intent is clear and indisputable and can accept that their intent has never effectively controlled policy and has in fact been explicitly repudiated by most presidents since Truman intervened in Korea without congressional authorization in 1950.

Reminded in a question period that he had defended Truman's step at the time, he responded that he had grown up in the 1930s believing that Congress was unfit for foreign-policy participation and presidents knew best, but that he has ''repented.'' This liberal Democrat now believes that conservative Republican Sen. Robert Taft, who protested Korea as ''a Truman war,'' was right and that Congress should order American policy.

Well, not many of us will be shocked at this demonstration that circumstance and time affect whether one believes that Congress is unfit and presidents know best, or the other way around. Fitting political ardor to constitutional principle has never been easy.

I like it, nonetheless, that Schlesinger does not flinch from the difficulty -- the embarrassment -- of trying to make a match. This is how a critic of ''original intent'' doctrine finally comes to rest his case on something he identifies, I think plausibly, as ''the deeper intentions of the Framers.''

The Constitution, says Schlesinger, commands a ''partnership'' whose terms may vary according to political and geopolitical pressures of the day: ''That, in my view, is the way it should be. The essential questions of foreign policy belong in the political arena. They must be argued out before Congress and the electorate. The salient question must be the wisdom of the measures proposed . . .

''Congress must understand that it cannot conduct foreign policy. The presidency must understand that no foreign policy can last that is not founded on popular understanding and congressional consent . . .

''The chief restraint upon those who command the physical forces of the country, in the future as in the past, must be their responsibility to the political judgments of their contemporaries and to the moral judgments of history.''