Early this month a top aide of French President Valery Giscard d'Estaing slipped inconspicuously into the headquarters of Gen. Alexander Haig, the NATO commander, for several hours of quiet speculation on Soviet intentions and U.S. reactions, particularly in Africa.

One day earlier, the defense minister of Japan - a country beyond the reach of NATO - undertook a similar journey to Mons, Haig's headquarters. Other leaders of the world's democracies, including the prime minister of Australia, have detoured to Mons during European sojourns to confide their worries to Haig.

A common theme - concern over expanding Soviet power and confusion over the Carter administration's foreign policy - motivates these democratic leaders. Indeed by forfeit, Haig has become a Western symbol of will and determination, clearly justifying President Carter's decision last year to retain him as NATO commander.

But while justifying that decision, the parade of Western policymakers to Haig's headquarters suggests agonizing doubts about Carter's inner convictions over the Soviet question. The president's reactions are marked by too many ambiguities and divided counsels.

Haig's prestige among democratic leaders of U.S. allies tells more about this disarray in Washington (as perceived by those leaders) than it does about Haig. Once attacked as a "political general" in an effort to block his appointment to NATO by Gerald Ford, Haig has become a kind of anchor to windward for worried Western Europe. As one powerful politician in the alliance (a West German) told us, "haig is the Linus blanket for all of us who cannot read the signals from Washington and need comfort."

President Carter seems to understand the value of Haig as a security blanket. On his last visit here in early January, Carter bluntly confided to one NATO official that he had had "some doubts" about Haig when he became president. Now, however, a Carter-Haig alliance has formed around NATO, resulting in at least one singular achievement in the president's otherwise shifting course on meeting the Soviet threat: strengthening the alliance along lines first recommended by Haig before Carter took office.

The beginning of this strengthening effort against escalating Soviet power in Central Europe came with the report of NATO deficiencies by Sens. Sam Nunn (D-GA.( and Dewey Bartlett ?R-Okla.) in 1976. Their prescription, strongly promoted by Haig, began with the stuff that counts most and comes hardest: increased financila assessments on all NATO countries. Today the United States, West Germany, Great Britain, Belgium and Holland are all meeting or exceeding higher levies agreed to with Carter in the spring of 1977.

By next fall, a new U.S. brigade for the central German front, termed essential in the Nunn-Bartlett report, will begin deployment. U.S. strength is now at its highest level - about 300,000. There is reason to believe the specialists here whi claim that the NATO "tail" of support troops has been shortened, with a consequent increase in the NATO "tooth."

Jimmy Carter has accepted the most important NATO proposals for dealing with possible surprise attack - the Warsaw Pact "blitzkrieg" that Nunn and Bartlett warned against. Upgrading communications and detection technology, vastly expensive, is now going forward with a 1980 completion date.

In February, Haig informed Secretary of Defense Harold Brown that the NATO high command was not being consulted enough by Washington - not only on Europe but also on Soviet moves in Africa and elsewhere. He is now being consulted. Brown informed him in advance of the president's astounding decision to cancel production of the neutron warhead. Haig's NATO colleagues told us that Haig warned Brown to tell Carter that cancellation would have devastating - but, for political reasons, undercover - anti-Carter repercussions in West Germany and some other European states.

Haig's intercession was a key factor in switching Carter's decision from cancellation to postponement. Haig's intercession was also crucial in the president's decision to authorize the use of Air Force troop carriers in the Angola-Shaba crisis.

Even with the neutron non-decision, Haig praises Jimmy Carter for playing top sergeant in NATO and whipping the alliance into better fighting condition. As to Carter-administration moves elsewhere in response to Moscow's worldwide operations, Haig's NATO colleagues say he privately shares many of their own doubts - the same doubts about Washington's strategy, or lack of it, that obsess those leaders of the world's dwindling democracies now beating a path to his door.