Suppose you were a member of the Politburo, sitting in some corner of the Kremlin contemplating the mess you've made in Afghanistan, when the news came in: the United States is not going to register its young men for the draft, after all. A court has said that leaving out women is unconstitutional.

Would you have heaved a sigh of relief?

And then when word arrived a few days later that -- hold everthing -- the registration is going forward on schedule while the Supreme Court reflects upon the constitutional issues, would you have flinched and started looking for a way out of Afghanistan?

Or would you merely be half-baffled and half-contemptuous of the U.S. government's capacity to convey a consistent, coherent purpose?

The latter, would be my guess; the more so as the Soviets ponder the words of the opposition party. Simultaneously, its leader, Ronald Reagan, denounces the registration plan as well as peacetime conscription, while the party platform tells the nation it faces "the most serious challenge to its survival in the two centuries of its existence."

Surveying all this, how could not only the Soviets but our allies be anything but reinforced in their doubts about U.S. intentions and resolve?

What is an American citizen to make of it? The same thing, it seems to me -- with perhaps a little more understanding of the complexities of the system and, accordingly, a somewhat better sense of what went wrong and what it would take to set it right. Indeed, the saga of draft registration from its inception in President Carter's State of the Union message in January constitutes an almost perfect case history for a seminar on the problems of making foreign policy, American-style.

The trouble started -- but by no means rests entirely -- with the State of the Union proclamation of a Carter Doctrine: "An attempt by any outside force to gain control of the Persian Gulf region . . . will be repelled by any means necessary, including military force."

For those fighting words, Carter needed a quick cutting edge. Draft registration was to be the chosen instrument. But registration only becomes a measure of will and willingness to sacrifice to the extent that is seen as a first, warning move toward mobilization. By itself, it makes only a marginal contribution to combat readiness.

And the more registration looks like a first step, the more it raises the explosive political issue of the logical next step: a peacetime draft. The result was, inevitably, a loud protest. Anti-draft movements mushroomed. A filibuster briefly stalled the Senate. Knuckling under to election-year pressures, Congress invited a court challenge by amending the White House proposal so as to exempt women.

Meantime, the Carter administration was insisting with increasing fervor that registration was a mere matter of convenience, and that actual conscription was nowhere in its plans. So much for the Carter Doctrine's quick cutting edge. What remained of it last week was removed by a Philadelphia federal court ruling that not only the registration program but the draft law, of which it is a part, is unconstitutionally discriminatory by virtue of its exclusion of women. Only a stay of that ruling by Associate Justice William F. Brennan Jr. allowed registration to begin this week while the Supreme Court reviews the case.

What we are left with is not just a scrambled signal to the Soviets, which would be serious enough.The net effect of the interplay of executive, congressional and judicial action on this particular exercise in the conduct of foreign policy is actually worse than nothing. For there is now a dark and dangerous legal cloud over this country's capacity in some sudden emergency to expand its military manpower by use of the draft.

Ultimately, the cloud will be lifted whenever the Supreme Court gets around to ruling. But meanwhile, constitutional law experts are putting it about that eligible registrants (men born in 1960 and 1961) can probably defy the registration law fairly safely, until a final Supreme Court judgement is handed down.

If the high court reverses the three judges in Philadelphia, you could argue that no permanent damage will have been done. But if it finds that the exclusion of women is discriminatory there will be no stand-by draft law on the books and no registrants on the rolls, because the names and addresses collected this week and next would presumably have to be expunged.

Congress would then have to write a whole new Selective Service Act, reviving the debate not only on registration and peacetime conscription but on the inclusion of women as well.

The unhappy moral of this story would seem to be that even at times of high crisis, there are some warning signals this country simply can't send to an adversary -- above all in an election year.