Even before West German Chancellor Helmut Schmidt arrived here for talks with President Reagan, Defense Secretary Caspar Weinberger, in secret talks in Europe, had laid down a hard bargaining position to prevent the Germans from reneging on their commitment to modernize NATO's nuclear weapons.

Weinberger's warning: If the socialist regime in Bonn fails to follow through on modernizing NATO's antiquated nuclear arms and on building up the alliance's conventional strength, there will be harsh consequences; the Reagan administration will have no obligation to fulfill its pledge to negotiate reduction of Central European nuclear arms with the Soviet Union. n

After that threat, the Germans acquiesced -- at least for the time being. Thus, Weinberger's debut May 12 at the NATO meeting in Brussels as the West's foremost defense official proved a success.

He cunningly turned the tables on West Germany and some other members of the Western alliance. They are exerting pressure on Reagan to get the nuclear arms talks started with the Russians, but they drag their feet on the defense buildup.

Although the Weinberger threat was couched in diplomatic niceties, behind his calm words the chilling force of his argument was unmistakable. Sitting at the long table in NATO's Brussels headquarters, Weinberger did not like what he was hearing from West German Defense Minister Hans Appel. Appel seemed to be hedging on the long-established pledge for a 3 percent annual increase in NATO defense spending.

"Words, words, words," Appeal complained to Weinberger. "All we do is talk about words and formulas." Instead of "words," said Appel, "performance" was the important yardstick.

That sounded to Weinberger like the overture to bad news from the Germans: a refusal by Appel to be pinned down to specific spending for strengthening NATO at the 3 percent rate. Such German reluctance conflicts with U.S. intelligence estimates that the Soviets have been escalating their Warsaw Pact spending at least by 4 percent a year since 1970.

Weinberger's suspicions proved accurate. When Appel offered a skimpy West German spending figure with a bonus the second year if his figures were accepted, Weinberger rejected it out of hand. Quite apart from its inadequacy for stockpiles and conventional arms, Weinberger told Appel that his offer would leave a dangerous shortfall in the nuclear modernization program -- particularly for the ground launched cruise missiles to be deployed in West Germany.

It was at this point that Weinberger, speaking quietly, issued his warning: If the Germans hedged on their commitment to nuclear modernization, they must understand that Reagan would be free to hedge on his pledge to start arms talks with Moscow.

Weinberger, backed by NATO Secretary General Joseph Luns, kept pressing Appel in a tense session that ran far beyond scheduled adjournment into the late evening. Here was Mr. Tough Cop skillfully following up Secretary of State Alexander Haig's Mr. Nice Cop routine at the Rome meeting of NATO foreign ministers two weeks earlier; in Rome, the United States agreed to start nuclear talks with Moscow late this year.

Haig's performance laid the foundation for Reagan's unfolding NATO strategy: agree to nuclear talks while NATO rebuilds its nuclear aresenal. Weinberger added the superstructure in Brussels: make clear that the promise to talk with Moscow was contingent upon West Germany's performance on nuclear modernization.

Before recessing that evening, Appel had increased the West German contribution by almost one-third. He also had agreed to NATO cruise missile spending of some $280 million to start the dangerously overdue process toward deployment of the ground-launched, nuclear armed cruise missiles.

The May 12 session ended in harmony. It put the United States and West Germany on as solid a footing as possible, given the contradiction between the determination of the Reagan administration to restore Western defenses againt the Soviet challenge at any cost and the tendency of the potent left wing of Schmidt's party to deal with Moscow at any cost.

Chancellor Schmidt's visit to Washington did nothing to undermine the accomplishments of Reagan's men in Rome and Brussels. Still, the threat to the Washington-Bonn alliance has been disminished only marginally. It will take astute state-craft to reverse the centrifugal forces that are pulling the old allies apart.