"If you don't think of it in cold war terms, but rather as a far more subtle challenge," says an American diplomat with many years of experience in Europe, "then the struggle for Germany is just beginning."

West Germany, the United States' most important military and economic ally in Europe, remains firmly tied to the West and the North Atlantic alliance.

What the diplomat is talking about, however, is a new challenge to politicians and strategists in Bonn and Washington posed by a steadily emerging shift for the better in relations between the Soviet Union and West Germany.

Although some of this is viewed as natural, the long-term implications are attracting the increased attention of policymakers in Washington, at NATO headquarters in Brussels and, presumably, in Moscow and Peking.

The development has taken on added dimensions because relations between Bonn and Washington have not been as cordial under the administration of President Carter as in the past.

In addition, a major debate is now developing here over how the West should respond to the challenge of increased Soviet medium-range missile forces targeted on Western Europe. For West Germany, finding an answer that fulfills Bonn's commitment to NATO but does not unduly upset the Kremlin is a delicate task that already is causing serious splits in the ruling Social Democratic Party.

For all the day-to-day comments, however, experienced observers here regard the Moscow-Bonn developments as something more fundamental.

"I don't see this so much as mistrust of Carter," one U.S. official said. "Regardless of the shape of U.S.-German relations, German-Russian relations would be important even if Lincoln were president."

The Soviets, of course, effectively control East Germany, a third of prewar Germany.

"So while West Germany's commitment to NATO is deep, its commitment to Germany, meaning all of it, is also deep."

Furthermore, "the big change in attitudes" thus far, another official said, has been on the part of the Soviets.

The shift began about 18 months ago, specialists here believe, and was both formalized and heightened by the visit to Bonn last May of Soviet President Leonid Brezhnev. During that visit, new long-term economic cooperation agreements were signed. Since then, longstanding autobahn agreements linking West Germany with Communist East Germany were achieved, with Kremlin approval. The flow of ethnic German refugees from Eastern Europe also continues, also with Moscow's blessing. The official put it this way:

"The Russians used to hammer away at the West Germans as neoNazis, warmongers and as an unstable element. That was the standard portrait for the postwar period and it continued even through the 1971 treaties" that normalized relations between the two former enemies.

"Now, they have clearly eased up, even assessing the relations publicly in positive terms. They seem to have taken off the curse that they put on them."

Are the Soviets trying to loosen or neutralize Bonn's crucial ties to the West and will Bonn, over time, respond?

At the moment, U.S. specialists here tend to see the Soviet moves as tactical probes, meant to explore possibilities, rather than as any grand strategy.

Some diplomats believe that the Soviets are fascinated by the extent of German criticism of the United States in the past year and the growth in political clout of West German Chancellor Helmut Schmidt. While the Soviets probably have no illusions about pulling Bonn from the West, they undoubtedly would like to probe the U.S.-German relationship further.

The West Germans, the Soviets know, are uneasy about whether the Carter administration can juggle its policies toward Moscow and Peking without alienating the Soviets. The Bonn government has made clear that Moscow has priority in its relations with the major Communist powers.

In the short term, Moscow's more pleasant tone toward Bonn could affect West Germany's approach to the next round of U.S.-Soviet strategic arms talks, to deal with questions of European security.

"If the Soviets' positive attitude toward Bonn lingers, if you had 10 years of no pressure and further agreements, maybe it could make a difference" in fundamental West German attitudes "by making the Russian threat seem different," a specialist said.

Yet the Soviets, he added, must offer something to Bonn. In this view, there is only so much relaxation of tensions with East Germany that could be offered by the Soviets without stirring up the East Germans -- who are sensitive to changes and potentially a serious source of unrest for Moscow. In addition, relaxation in East Germany is apt to produce effects throughout Soviet-controlled Eastern Europe.

The Soviets, according to a common joke here, live in the only major Communist country surrounded by hostile Communist states.

Indeed, another diplomat says, the main thrust of the Soviet warming toward West Germany may be defensive. "The Soviets probably feel weak in Eastern Europe these days and while the West Germans can't really do anything about that, they could prove to be a stabilizing factor by perhaps throwing their weight on the side of stabilization in a crisis or at least not magnifying the differences" between East and West, he said.

As for Bonn's reaction to Moscow, opinion is divided.

One senior U.S. official says Bonn has been realistic about the Soviet advances, aware of the Soviet constraints and West Germany's safety in the West -- and not entertaining "romantic notions."

Others, however, describe what they sense as an "attitude of overindulgence" toward Moscow these days. Schmidt has gone out of his way on numerous occasions to promote the accomplishments of the Brezhnev visit. Some critics suggest that he has given the Soviets more credit than they deserve.

At the top of the Bonn government, however, officials see the improved climate with Moscow as something that keeps detente alive and benefits the Western alliance.

West Germany is also Moscow's largest trading partner in the West, and the Kremlin may expect further economic consideration in exchange for improved relations.

Leading West German officials stress, however, that good relations with the United States and a firm commitment to NATO remain a cornerstone of Bonn's policy that cannot be changed, despite superficial and occasional flaps.

The reason, according to one official, is that while West Germany is perhaps closer to the Soviets than anyone in the Western alliance, West Germans also have closer and more personal knowledge of living conditions in Eastern Europe and thus there is no question what they would choose.

"So I would not foresee any change within the next generation," the official said.