As the Reagan administration and the Soviet Union prepare for their first major diplomatic meeting later this month, a markedly different mood is evident on the opposing sides.

For the new administration, the occasion is an opportunity to show the Soviet Union its determination to reverse what the United States sees as a dangerous shift in the su News Analysis News Analysis perpower balance. For the Kremlin the meeting is a chapter in an ordeal to be endured until an unexpectedly assertive government in Washington is deflected from its goals by internal and external pressures.

Soviet sources concede in private that they initially underestimated how long it would take the Reagan administration to accept "world realities" and shift course. American strategists say that the administration has adjusted its initial campaign-style rhetoric to "the real world," and it is the Soviet Union that must now recognize a new stage in relations between.

As a result, neither side expects any significant shifts in position when Secretary of State Alexander M. Haig Jr. meets with Soviet Foreign Minister Andrei A. Gromyko in New York, Sept. 23 and 28.

Neither nation claims that the huge gap between them is the product of misunderstanding. Haig himself has said bluntly, "I think the problem is not communication. The problem is that the Soviet leadership, thus far, has not liked what they have heard from this administration."

Haig, with his penchant for applying military terminology to diplomacy, has invoked a battlefield characterization for his talks with Gromyko: "a meeting engagement," likely to produce "some rather stiff exchanges."

If Haig's metaphor creates the image of a titanic struggle on the edges of the United Nations General Assembly session, however, it is overdrawn. His Soviet counterpart, Gromyko, openly wearied of gladiatorial contests decades ago, although he will still dutifully engage on demand.

For Gromyko, now 72 and the longest survivor in superpower diplomacy (starting in l939), the Reagan administration's demands undoubtedly evoke Dean Acheson and John Foster Dulles and their reliance on what the Soviet Union still denounces as "positions of strength." But the Reagan administration does not shrink from using military power to stiffen its diplomacy; it proclaims that objective.

For the first time since the early years of the Kennedy administration, the Reagan administration explicitly has committed the United States to the use of "strength" in a "two-front" policy. The dual objectives, as described by Haig, are to create "barriers to aggression" by "renewing American strength," and secondly, to offer "incentives for restraint" if the Soviet Union will "respect international norms of behavior."

No American diplomat on his own would express the challenge to the Soviet Union as baldly as President Reagan stated it in Chicago last Wednesday. He declared that unless Soviet leaders accept "legitimate" and "verifiable" reductions in strategic weapons, "they will be in an arms race which they can't win."

Reagan expressed a similar position in his election campaign; to that extent he was telling the Soviet Union nothing new. He was not threatening the Soviet Union, Reagan said last week as he addressed a Republican fund-raising dinner. But even though he stated policy in terms of a choice for the Soviet Union to make, his open reference to an arms race is readily convertible by Moscow into anti-American ammunition in the world propaganda forum. That is the forum in which the Soviet Union is concentrating its efforts to thwart the Reagan strategy.

Military power is critical in the Soviet Union's own multitrack policy to extend its reach in the world. But the Soviet Union never openly concedes that "strength" is at the core of its policy; its military power is always portrayed as intended solely to checkmate opponents bent on its destruction.

At the outset of the Reagan administration, the Soviet Union dismissed American calls for reshaping the superpower military balance as the impractical talk of newcomers. But as the administration's defense spending plans gave forcefulness to its commitments, Soviet alarm has increased.

It is time to end the debate about "whether there is or is not a policy" in Washington, Soviet commentators declared in a recent Moscow television discussion, for "there is a policy and it is being implemented."

Assaults on the American position have been carried to the highest levels of official Soviet indignation, in some of the strongest language in the Marxist vocabulary. Soviet Defense Minister Dimitri Ustinov, for example, has flayed the United States for "unconcealed anti-Sovietism," "monstrous deception and massive disinformation," and for engaging in a calculated attempt "to undermine" the "national economies" of "the U.S.S.R. and other socialist countries" by launching "an unprecedented arms race."

The Soviet Union is gambling in part, as one Soviet commentator expressed it, that "even the rich United States, which is actually not so rich, is in danger of overstraining itself beneath the weight of all these new military programs." The Kremlin is bound, therefore, to draw encouragement from the reductions the Reagan administration is required to make in its original defense plans. But the Reagan administration has reached for multiple pressure points on the Soviet Union.

Beyond military expansion, a major point of the administration's Soviet strategy is its sharp downgrading of arms control as "the political centerpiece" in the superpower balance.

In Haig's words, "We overestimated the extent to which the Strategic Arms Limitation Talks would help to ease other tensions." As a consequence, he has said, the United States restrained its own defense programs, while the Soviet Union was left free to invade Afghanistan, support the Vietnamese invasion of Cambodia, and project its power into Africa. Now U.S. arms control policy is geared to "the whole context of Soviet conduct worldwide," with arms control treated as only "one element" in total American foreign and defense policy.

The Reagan administration, which rejected the SALT pact to limit intercontinental weapons negotiated by the Carter administration, does not intend to produce its own stiffer terms for limiting these weapons until spring. Before then, the administration has told the Soviet Union, it wants to explore with Moscow basic changes in verifying and monitoring controls on nuclear weapons.

One level of arms control will be high on the agenda when Haig meets Gromyko.

The United States is committed, by agreement with its Western European allies, to negotiate limits on American and Soviet medium-range nuclear weapons in Europe. This is a condition for deploying American-built Pershing II and cruise missiles, to counter Soviet SS20 missiles already in place and targeted on Western Europe.

These U.S.-Soviet negotiations are scheduled to begin between mid-November and mid-December. Soviet policy now relies heavily on using these negotiations, and other portions of the emotional and complex arms control debate--newly inflamed by dispute over neutron weapons--to build a backlash in Western Europe against Reagan administration strategy.

The administration's ability to sustain allied unity on deploying and limiting nuclear weapons, therefore, has become an early, critical test of its entire East-West policy.