When the first of four Soviet naval vessels slipped quietly into Marxist Mozambique's main port Feb. 19, post-detente U.S.-Soviet rivalry notched up to a new peak with the Kremlin serving notice it would trump President Reagan's Caribbean ace.

This shows how hard it is for the Reagan administration to conduct a forceful, catch-up foreign policy from a position of relative military weakness. The new policy of boldness in the Caribbean, overdue after years of futile courtship of Cuba's Fidel Castro, has triggered a heightened Soviet challenge in even more important regions where the Soviets hold the high cards.

Such a Soviet response has been predicted by worried pro-Reagan hard-liners in Congress. While applauding the president and Secretary of State Alexander Haig for long-overdue drawing of the line, they have been concerned about too much talk.

While making no public criticism of a toughened Caribbean policy they applaud, aimed at ending Soviet-backed arms shipments to El Salvador, Reagan's hard-line critics have privately cautioned administration officials that too much talk could promote Soviet responses. These would come with the United States so far behind in the arms race that countermeasures would be difficult.

Arrival of the Sverdlov-class light cruiser Suvorov in the port of Maputo Feb. 19 showed the accuracy of these warnings. The Suvorov was shortly followed by three other Soviet navy ships: a Kashin-class destroyer, a frigate and an auxiliary vessel, all detached from the Soviet Indian Ocean fleet.

That formidable naval power has now been added to a rapidly increasing number of military and economic "advisers" from the communist bloc now numbering more than 5,000 well over twice those working in Mozambique less than three years ago. These include 2,500 Soviet and East German military specialists who, with 1,500 Cubans, are now training Mozambique military officers to fly MiG17s, to operate SAM3 anti-aircraft missiles and to drive over 200 tanks.

The stakes in the southern Africa power game, where Mozambique is one of the high cards held by the Soviets, are breathtaking: control of the most concentrated mineral wealth anywhere in the world on land; and control of the oil sea lane from the Persian Gulf down the east coast of Africa. Europe gets 90 percent of its imported oil through that route and 70 percent of its imported minerals from southern Africa.

The Soviet game in southern Africa is to win what Haig called "the era of the resource war" in testimony to a House subcommittee last fall, before he became secretary of state. He said that if "future trends, especially in southern Africa, result in alignment with Moscow of this critical resource area, then the U.S.S.R. would control as much as 90 percent" of key minerals vital to the economy of the United States, Western Europe and Japan.

The sudden arrival of four naval vessels in Maputo, together with the steadily expanding contingent of communist "advisers," shows that "alignment with Moscow" is moving ahead fast. Indeed, some diplomats here believe that the Soviet bloc pushed hard for the unusually harsh edict of the Mozambique government that expelled four American Embassy officials last week on spy charges. That followed a daring South African raid Jan. 30 on the Maputo headquarters of the anti-South African National African Congress.

Using its 1977 friendship with Mozambique, the Soviet Union is believed to be leaning on the Maputo government to invoke Article 9 of the treaty, pledging Soviet aid to eliminate any "threat" to peace.

South Africa, along with its racial policies, is the cement used by the Soviets to bind Moscow to black southern Africa and create a band of Soviet power across the southern tip of the continent from Mozambique to Angola. Hoping to close the final link in that band, the Soviets finally won full diplomatic relations with Zimbabwe last month.

Dealing with this Soviet subversion in an area as important to the industrial democracies as the Persian Gulf is unavoidably more challenging than imposing a long-needed Caribbean quarantine now planned by the new administration. It requires military strength -- the existence of real power to compete with the Soviet Union.

Reagan has asked Congress to give him that, but he is far from possessing it. Until he gets it, his conservative critics will praise his resolve but continue to worry about too much loud talk in America's back yard.