An unfortunate choice of words by President Carter is responsible at least in part for Moscow's misinterpretation of - and angry reaction to - his speech last Friday on defense strategy. The first official Tass commentary said that the president had announced a "major reassessment" of U.S. strategy, and Moscow understood that to mean Carter had almost turned the clock bck to the cold war. It detected "a shift of emphasis" from Carter's policy of negotiations, detente, and limitation of the arms race "to a policy of threats and of building up tensions."

Carter had indeed said that "we have recently completed a major reassessment" of national defense strategy. What he failed to make clear, however, was that the reassessment was completed eight months ago.

It was last August that Carter issued his Presidential Directive 18, a five-page document designed to serve as a guide in the formulation of foreign policy and national security. But the Kremlin's spies have evidently failed to get hold of PD 18. The Soviet reaction to Carter's speech suggests an assumption on Moscow's part that the president has enunicated a completely new and much tougher U.S. policy, and that the Kremlin had to respond accordingly.

What is new is Carter's public presentation of the main conclusions of PD 18, though without any reference to the original document. First, he said, the Unted States would maintain the strategic nuclear balance. Second, it would work with NATO to strengthen Europe. Third, it would build up forces "to counter any threats" to the "vital interests" of the United States and its friends in Asia, the Middle East and other regions.

The first of those is now new, since the maintenance of the strategic balance has been the objective of all previous administrations. The second, the European defense policy, is half new. Previous administrations have paid lip service to it, but failed to carry it out fully because of their commitments in Vietnam and other preoccupations. It is in the third category, which used to be known as peripheral, that the Carter administration has made a real change.

Carter says that the United States has "important historical responsibilities" in East Asia, the Middle East and the Persian Gulf. While his preference is to use international agreements to "minimize the threat of conflict" in those and other areas, he goes out of his way to tell the Kremlin that he is prepared to act if provoked. "We have the will," he says, "and we will also maintain the capacity to honor our commitments and to protect our interests in those critical areas." To that end the Defense Department is building up - "at my direction" - forces that could be quickly deployed to any area.

At a time when a Soviet-Cuban expeditionary force of more than 10,000 men was quickly deployed to Ethiopia, Carter's warning about the Soviet Union's "ominous inclination" to intervene with military power in local conflicts, and his determination to have the forces with which to counter that inclination, will not be lost on the Kremlin. But this is not a sudden response to the Soviet presence in the Horn.

PD 18 had, in fact, anticipated possible Soviet adventures of that kind and had therefore emphasized the need for a considerable improvement in the mobility of U.S. forces, and for the strengthening of conventional forces. That emphasis is based on the assumption - which U.S. policymakers don't acknowledge in so many words - that the United States can no longer rely on nuclear superiority, as it did in some previous crises. The assumption is that there will be considerable advantages to the side that can get its forces to the crisis area first - for the other side would then have to dislodge them.

It is the decision to do something about these problems that is beginning to introduce new elements into U.S. strategic thinking. But the Kremlin concentrated its attention on those remarks of Carter's that its says are "incompatible" with his claim that the United States wants to avert a nuclear war. The Kremlin was paying attention to the new tone of the president's speech, rather than to its substance.

Carter's theme was the need for U.S. strength in the face of Russia's growing power, and his intention was to assure both the Kremlin and his domestic critics that he would do whatever is necessary to meet that challenge. His speech was peppered with references to strenght, "the final protector of liberty." The chances of wa would be reduced by "demonstrated strength." The United States would "maintain strength equal to the challenges" facing it. Those who could destroy liberty were restrained by the knowledge that "those who cherish freedom are strong." And so on.

Moscow got the message. Most Western news reports ignored Carter's rhetoric and concentrated on the substance. But the Kremlin's analysts would no doubt have treated every one of Carter's flourishes with the respect they accord to their own leaders' assertions. Were they right to do so? Carter's speech had gone through several drafts, and both the hawks and the doves in his Cabinet had had a chance to suggest revisions. They did, and the last draft contained tough as well as mild versions of the key passage - as well as something in-between. Carter himself made the final choice - and he did not always choose the toughest.

Some of his domestic critics welcome the tone of his speech, but they are still waiting to see whether it will be translated into policy. Moscow, on the other hand, dislikes the tone - but assumes that it represents a new policy. In fact, the struggle for Carter's soul between the hawks and the doves in his entourage is still going on, although the hawks have made some progress, and will make more unless Moscow moderates its conduct - which is the real message Carter intended to convey to the Kremlin.