Unraveling of the Western alliance is the justification given by Ronald Reagan's national security advisers for making Gen. Alexander M. Haig the leading prospect to become secretary of State in a Reagan administration.

Reagan's foreign policy team, headed by Richard V. Allen, sees a major asset owned by Haig in any future dealings with Western Europe: intimacy with West German Chancellor Helmut Schmidt developed by Haig during nearly five years as supreme commander of NATO.

Reagan is alarmed at signs of a breakdown in the alliance, most lately President Carter's sharp letter to Schmidt on the eve of the Venice summit. Restoration of strength and harmony, particularly between the United States and West Germany, is Reagan's first step to any workable Western strategy in dealing with the worldwide Soviet offensive.

Therein lies Haig's allure as Reagan's instrument to make the industrialized, democratic world whole again. In the words of one high-level West German leader: "Naming Haig secretary of state would do more to revive the alliance than any single step I can think of."

It is the Reaganite thesis that Europe is the key to Western survival and that Schmidt is the key to Europe. That thesis, shared by many American foreign policy experts outside the Reagan camp, assumes a big Schimdt victory in the October election.

This points toward Al Haig as a natural Reagan agent for rebuilding the alliance. During Schmidt's last visit here, a March 6 dinner given for the chancellor by banker Rockefeller in his Manhattan townhouse was conspicuously marked by a long, private chat in the library between Schmidt and Haig. When Haig underwent heart surgery at Houston Presbyterian Hospital in early April, Schmidt sent him a personal letter urging a speedy recovery.

To Schmidt, Jimmy Carter has been an uncertain partner with a dismaying tendency toward sudden policy swerves that have left Western Europe numbed and angry. Reagan is unknown in Europe; Haig at his side would provide reassurance. Arriving unwanted in Brussels in November 1974 for his NATO assignment, fresh from duty as Richard Nixon's White House chief of staff, Haig shed the Watergate taint quickly. In a few months, he had become a rallying point for the Western alliance.

The one impediment to Haig, however, could be watergate. One Reagan insider told us "some stuff" on a White House tape, not available to the public, could hurt the general by revealing his reaction when Nixon first briefed him on the Watergate scandal. If this actually showed Haig knowing more than he later admitted, it would be fatal. But nobody is claiming this, and the tape may never be played publicly.

But Reagan's advisers think Watergate is out-weighed by Haig's assets. Haig Kissinger's lieutenant on Nixon's National Security Council staff, could be a bridge between Reagan and Kissinger. Under no circumstances will Kissinger be invited into the Reagan administration; nevertheless, Reaganites do not want an open rift with him.

Reaganites also credit Haig for his connection with the old East Coast Republican establishment, or what is left of it. Such Republicans backed Haig's abortive campaign last year for the presidential nomination. Haig in the end declined to run for lack of support and instead became president of United Technologies.

Another source of support for Haig is the American Jewish community, especially the American-Israel Public Affairs Committee. American supporters of Israel believe his standing with European leaders, particularly Schmidt, could influence them toward a friendlier attitude toward Israel.

But the main driving force of Haig's Jewish backing is identical to Reagan's own reason for seriously considering him as his secretary of state: a realistic view of Soviet power and expansionism in the world. As such, Haig as secretary of state would fit a Reagan world view that is more coherent than his critics are willing to acknowledge.