Zbigniew Brzezinski's quiet recruitment of a tough-minded Air Force general as top defense strategist for his National Security Council staff, far from being a routine staff addition, signals growing influence and new self-confidence for President Carter's often beleaguered national security advisor.

The new aide is Air Force Maj. Gen. Jasper Welch Jr., considered one of the Pentagon's top experts in the eerie science of U.S. Soviet military balance -- a balance now clearly tipping toward outright Soviet superiority.

Brzezinski's decision to hire Welch was urged by Dr. James R. Schlesinger, the one-time Nixon-Ford defense secretary and until recently Jimmy Carter's energy secretary. It raises important -- and as yet unanswerable -- questions: as Carter moves toward his final first-term year, does he now begin to share Brzezinski's forebodings about the worlwide threat of growing Soviet power? Or does Brzezinski's new confidence, symbolized by the recruitment of Welch, simply reflect a Carter political tactic to look hawkish to get SALT II through the Senate?

Although there is no answer yet, the new staff role being readied for Welch is certain to produce a much sharper focus within the NSC on overall stragic policy -- a focus long desired by Brzezinski. It will also give NSC a more persuasive voice within the administration.

It is not generally understood even on Carter's Washington that Brzezinski's power to place experts of his own choice in NSC staff positions has been limited until recently. When he accepted the top NSC staff job, foreign policy was in the hands of the McGovernite staffers who controlled appointments to major second-level posts in the State and Defense Departments -- and in Brzezinski's own NSC.

For example, Brzezinski had no say in the choice of his No. 2 man, David Aaron, a one-time NSC technician under Henry Kissinger who became foreign policy adviser for Sen. Walter F. Mondale. Vice President Mondale dictated his appointment to the NSC.

The Brzezinski-Aaron relationship has been cool during the whole Carter presidency. Recently, in another display of his rising self-confidence, Brzezinski quietly reduced Aaron's power within the NSC, stripping him of authority over several major functions. U.S. economic policies toward Europe and the Third World were handed over to Henry Owen.

Until recently, Aaron's political clout within the NSC had been profound, thanks mainly to Mandale's patronage. It was Aaron -- in cooperation with Mondale and the dovish transition team -- who installed a black law professor, Henry Richardson, as chief of Brzezinski's Africa policy section. Richardson espoused the hands-off, pro-Third World policy in black Africa preached by then-U.N. Ambassador Andrew Young and Assistant Secretaries of State Richard Moose and Anthony Lake, major Brzezinski adversaries. Brzezinski managed to ease Richardson out several months ago, after Brzezinski had gone public against Fidel Castro's blatant intervention in foreign countries as Soviet surrogate.

But despite the growing public -- and therefore political -- awareness that the administration's soft-line approach to the Soviet offensive was clearly not working, Brzezinski continued to move furtively and cautiously, fearful that anti-Brzezinski hostility within the bureaucracy -- plus the president's tendency to come down simultaneously on both sides of tough foreign problems -- might isolate and destroy him.

That period seems now to have ended. Brzezinski's self-confidence in pushing him to bear down harder with his own more pessimistic views about the U.S. decline. He gained crucial yardage during the recent Cuban crisis over the Soviet combat brigade, even though it was the president who backed down.

Marshal Shulman, the State Department's Soviet expert, had predicted to Secretary of State Cyrus Vance -- and thus to Carter himself -- that Soviet Foreign Minister Andrei Gromyko would be "flexible" in dealing with Carter's demand that the status quo be changed.

But Brzezinski and his own Soviet experts took a more realistic and, as it turned out, more accurate view: that the Soviets, reinforced by their world-wide military power, had no intention of giving in to Carter or saving his face.

"Zbig had nothing to do with the sorry way brigade crisis was handled at State," one White House insider told us. More important, Brzezinski argued hard against a proposal that Carter publicly attack Castro after the unsatisfactory conclusion of the crisis over the Soviet brigade. Insiders insist that Brzezinski viewed this proposal as a bluff that could have only one effect: build up the communist dictator of Cuba. Carter took that advice.

Brzezinski wants fewer tough words and more real deeds on the old-fashioned assumption that the only way to deal with the Soviet world offensive is to be backed by adequate military strength.

The recruitment of a high-level Air Force general to the NSC is major move toward that goal. It signals the terminal point of what one aide calls the "understandable insecurity" that hobbled Brzezinski during the first 30 months. That could stiffen Carter's spine for the harder Soviets battles that lie just ahead.