Secretary of State Alexander M. Haig Jr., in a speech that appeared to set the stage for opening discussions with Moscow on reducing strategic nuclear arms, said yesterday that "nothing is to be gained by appearing to fear diplomatic discussions" with the Kremlin on this subject.

Contending that "important changes are taking place in the world and in the Soviet empire that may make Moscow more amenable to the virtues of restraint," Haig said, "we can no more solve our problems by avoiding the negotiating table than by resting our hopes on it alone."

In Haig's view, "Soviet prospects have dimmed" at home and around the world, with "Moscow's allies in deep economic trouble . . . the Soviet growth rate declining . . . and agricultural shortfalls persist." The United States, he said, therefore has "a historic opportunity in dealing with the Soviet Union," provided that this country maintains strong defenses.

"As a new generation of Soviet leaders emerges," Haig said, "we can signal the benefits of greater restraint." Although Haig's message appears to have been aimed especially at those now maneuvering for power in Moscow as the era of President Leonid I. Brezhnev draws to a close, officials said it was also meant for the incumbent regime in the Kremlin.

Informed sources yesterday said they expected President Reagan to make a major speech on East-West relations next month and that it likely will include announcement of a proposed starting date for the strategic arms talks.

Haig said the United States is "prepared to show Soviet leaders that international moderation can help them face painful domestic dilemmas through broader relations with the U.S. and other western countries."

At the same time, he appeared to challenge hard-line American critics of negotiations about the inconsistencies of their arguments. "We cannot claim that we are too weak to negotiate and at the same time insist that we are strong enough for a policy of all-out confrontation" with Moscow, he said.

Haig's remarks yesterday came during a speech to the U.S. Chamber of Commerce here that the secretary personally touted as "an important foreign policy address."

In that speech, Haig also delivered a strong defense of American alliances, but made no mention of the current imbroglio involving Britain and Argentina in the South Atlantic. He warned those calling for withdrawal of U.S. troops from Europe or trade barriers against Japan that, as Winston Churchill said, "The only thing worse than fighting with allies is fighting without them."

In a third major element of his speech, Haig also called for the West to "seize this opportunity" to develop better ties with leaders of developing countries of the Third World, even those with ties to communism.

"Marxist-Leninist ideology has often been the locomotive that brought them to power," Haig said, "but it has not become an engine for progress, . . . and many countries with direct experience of the Soviet embrace are quietly attempting to broaden their relations."

Haig's defense of the alliance comes as sentiment grows among some in Congress to pull back U.S. troops from overseas because of a view that the allies are not carrying a fair share of the common defense burden.

Haig said the allies "must develop a broader vision and sense of responsibility consonant with their interests and strength. They cannot expect the U.S. to carry the same share of the burden when our respective capabilities have changed and their own desire for influence has grown."

But the main thrust of his remarks constituted a sharp defense of the allies that undoubtedly will be welcomed in friendly capitals. Haig said, "It is high time that our dialogue proceed on the basis of fact."

Americans should not forget, he said, that the NATO allies "substantially increased their defense spending over the past decade while the United States was reducing its defense efforts." European members of NATO supply the highest percentage of non-nuclear air, ground and naval forces in Europe, he said.

While the United States must exert strong leadership, Haig said, "The allies must know where we are going if we expect them to go with us. Their policies, especially in dealing with the Soviet Union, reflect not only differing perspectives of Soviet actions but also a tendency to hedge their bets against American swings between detente and confrontation."

Although Haig said "we cannot pretend to lead unless we rally to our side those societies that share our values," his speech did not link the alliance issue to the Falklands. The administration, which is trying to mediate the dispute, has passed up all opportunities to take sides in public despite some criticism that the United States should be seen as clearly siding with London.