IN NO AREA of public policy did President Reagan's mandate appear more urgent a year ago than in foreign policy and defense. Previous administrations, he charged, had left the nation's prestige in tatters and its security in peril. He identified Soviet expansionism as the principal menace to American well- being and he promised, by "rearming" and by asserting American will, to contain it effectively. Iran's release of the hostages the day of his inauguration seemed to attest a general recognition that Washington was getting serious about power again.

A year later everything seems fuzzier. In foreign affairs, the early Reagan inclination to base policy on a hard, consistent ideological line has broadened to include--though more in deed than word--a more pragmatic readiness to accommodate anxious allies and domestic constituencies: to negotiate on arms control with those nasty, untrustworthy Russians, for instance, at the same time that he offers a serious strategic arms program. Some of the foreign nations considered to be most in need of a strong American anti-Communist embrace have shied away from it. Larger regional considerations have induced Mr. Reagan to take a little distance from countries --Israel, Taiwan and to a lesser degree South Africa --that had looked forward to special favor. In defense, meanwhile, the president faces immense pressures across most of the political spectrum to scale back planned spending increases.

The public seems to retain confidence in Mr. Reagan's stewardship, but after a year his aides keep having to explain why he should not be expected to master the fine detail. In some respects, he is no better a manager of policy than his much-abused predecessor.

It goes without saying that in various matters-- El Salvador is the leading case--Mr. Reagan has confirmed the apprehensions of liberals who voted against him. The stunning development, however, lies elsewhere. A year that began with release of the hostages ended with members of Mr. Reagan's core constituency complaining that he had kowtowed to Peking in the matter of selling new aircraft to Taiwan and that in Poland he had allowed what should have been exclusively a Soviet embarrassment to become the most serious Atlantic crisis in 30 years.

That things have not worked out according to plan is, to some Reagan advisers and supporters, intolerable. They offer three broad correctives. Some would recall the president to his ideological and political roots. Others would recast (even further) the structure, procedure and personnel on the national security side. Still others would consummate a grand design in one big speech. We can't get too excited about any of these courses. Something else seems to us more important.

Mr. Reagan came to foreign policy believing that the United States enjoyed a special dispensation allowing it to ignore the limitations--of resources, of politics, of knowledge and imagination and luck--circumscribing the conduct of other nations. He is beginning to accommodate all this in his thinking, and to do so exposes him to charges of inconsistency and falling away from principle--and also to wobbles and mistakes. But from the country's point of view, this is not the worst thing. The worst thing would be to resist learning and, where necessary, changing.

The true believers are aghast at what they take to be intimations of apostasy. But the results of a presidential education need not be the old establishment product. Mr. Reagan was elected because Americans thought they needed a stronger hand. If he has not yet been fully tested, nothing in his first year suggests he cannot be trusted in his turn at the helm. He has been looking for his own combination of firmness and flexibility, constancy of principle and sureness of style, and it is far too soon to say he cannot find it.