A more conventional president might have set foreign policy goals and organized a program to pursue them. But we have not had a president like that since Richard Nixon. More crusaders than managers, Jimmy Carter and then Ronald Reagan have proclaimed broad abstract goals and pushed on without a clear sense of relating means to ends. That is how Reagan got into the spot he is in now.

It is a special kind of spot, on the whole enviable, since it is the product not so much of failure as of the promise of success. Reagan has converted the thrust he brought to Washington and the new disposition to change evident in Moscow into the possibility of a better ratio of cooperation to conflict, steadiness to crisis, in Soviet-American relations. But he must now define and thereby cap the gains he hopes to achieve -- something his absolutism makes it hard for him to do.

In arms control, the question is whether the president clings to his maximal conception of space defense as an ultimate shield against nuclear attack. In regional disputes, he must decide whether to take the best arrangements available now or keep shooting for the perishable marginal advantage dear to the hearts of the conservative brigade. In human rights, the issue is at what point improvements in Soviet performance make Washington determine that further improvements are more likely to come from relaxing overt American pressures.

Hovering over everything is the grand issue of whether the Soviet Union is to be treated, as former American diplomat Charles Bohlen once put it, as country or cause: as a country as prepared as any other to accept the rules of the game of nations or as a place whose revolutionary impulse impels it to assert special interventionist privileges for itself alone. No final resolution is in the offing, but it is an impressive measure of the impact of Mikhail Gorbachev that he is driving even a dyed-in-the-wool believer like Ronald Reagan to consider the possibility of a change.

In his earlier White House days, Reagan did not have to address these tough questions. While he was distant from achieving some of his foreign policy goals, he could define them loosely and extravagantly. The administration's internal debate, such as it was, arose among people representing only different degrees of ardor. By his policy the president was provoking the question of whether the United States in its own way was country or cause, but it was a matter of pride to him, not embarrassment. The impulse impelling Washington to assert its privilege in international affairs was the impulse of universal liberty.

Much inquiry is still required into how considerations of time, design and opportunity combined to create the present extraordinary moment. The argument that Reagan's toughness did it has some merit, though not all that his champions claim. Ditto the argument that Gorbachev's suppleness or the peace movement's persistence did it. Conservatives don't relish considering that their role was simply to play a role, that of bad cop. They would prefer to be regarded as architects of a breakthrough.

But with breakthroughs come choices. In Afghanistan now, for instance, the administration pauses at the prospect of tapering off American support in order to ease and ensurea Soviet withdrawal, even thougha withdrawal -- as distinguished from a rout -- has been the right and nec-essary goal of American policy all along.

Reagan faces similarly fine tactical choices in respect to a number of his high-visibility military, counterinsurgency and other initiatives. With Moscow in a bargaining mood, should he push ahead with these programs or treat them as bargaining chips? It is a poor program that cannot generate a lobby to argue that it would be reckless to trade it away short of 100 percent satisfaction. An administration given to defining its goals open-endedly -- freedom, democracy, a non-nuclear world -- is bound to experience turmoil when it thinks of cashing in some of these programs, even when the price is right.

Vision in his own terms Reagan has shown in abundance, but it falls to him now to show self-discipline and a sense of proportion. For his pains he will draw the mutters and curses of some of the old faithful, but judgment, not passion, offers him his best chance of taking advantage of the new possibilities with Moscow, of not letting a rare moment slip by.