When the aging, infirm Soviet boss, Leonid Brezhnev, agreed to the invasion of Afghanistan last month, it was a triumph for the hard-liners in the Kremlin. It also marked the ascendancy of Soviet Defence Minister Dmitri Ustinov in policy councils.

It was Ustinov, more than any other member of the ruling clique, who persuaded Brezhnev to abandon detente and SALT II in favor of a return to the Cold War. According to U.S. intelligence sources, Ustinov was the mastermind of the Afghanistan aggression.

As an inside source told my associate Bob Sherman, "Ustinov is calling the shots on this one, and he is a real hardliner."

How did Ustinov maneuever into that position? A top-secret intelligence profile chronicles the 71-year-old commissar's simultaneous rise through the Soviet defense bureaucracy and the Communist Party apparatus over the years. s

Like his hard-line counterpart, in Washington, Zbigniew Brzezinski, Ustinov has no military background. Brzezinski was a university professor. Ustinov, clawing his way to power in a regime where eggheads are scorned, was a technocrat -- a specialist in armaments production.

"Ustinov's credentials in the defense industry go well beyond those of any other previous Politburo member," the profile states. "He has in fact spent . . . years shepherding the Soviet defense industry, literally from horse-drawn artillery to the nuclear rocket era."

Like Brzezinski, Ustinov owes his present high position to his ability to impress those in power, and like Brzezinski, he is answerable to his patron, not to the people. Ustinov was only 32 when he caught the eye of Joseph Stalin. Too young and low-ranking to have been cut down in the great purges of the '30s, he was named people's commissar for armaments in January 1941.

Five months later, the Nazis struck. The early disasters of the war could not be laid at Ustinov's door, and he was put in charge of supplies for the Red Army's ultimately successful drive against the Germans.

Ustinov remained virtually unknown outside the Kremlin in the years after World War II. But he was evidently working his way upward in the military-industrial bureaucracy. In 1965, he emerged as a candidate-member of the Politburo, and became Communist Party secretary in charge of the defense and space industries. In 1976, Ustinov achieved full membership status in the ruling Politburo and became Soviet defense minister with the rank of marshal.

"The defense industry is favored in the allocation of resources," the intelligence profile concludes, "but it is Ustinov who makes it produce in ways that other sectors of the economy cannot approach. Operating effectively within the world's most complicated bureaucracy; Ustinov has been an able manager and political tactician."

The report describes Ustinov's primary function in the Politburo as dealing "on a par with the professional military -- with the specific questions of resource allocations that demand the attention of the leadership."

In this role, Ustinov was a top adviser to Brezhnev at the SALT II summit in Vienna last summer. And while the secret report states clearly that Ustinov gave only lukewarm support to Brezhnev's policy of detente, it concludes that he viewed "arms limitations agreements and similar manifestations of better relations with the United States as somewhat less risky than does the professional military."

The intelligence report paints a picture of Ustinov as the ever-diligent manager who works hard "making sure that the defense industry's performance meets the leadership's expectations."

Specifically: "Ustinov's managerial style is activist and pragmatist. Typically, he works from 8 a.m. until between 9 and 11 p.m. weekdays, and puts in about four hours on Saturdays.

"When he is not pressed by urgent party business, Ustinov likes to pay early-morning visits to plants or design bureaus in the Moscow area, usually on short notice and often before the managers arrive."

U.S. intelligence analysts regard Ustinov as unlikely to succeed Brezhnev in the top party position. "The narrow focus on defense industry throughout his career, together with a lack of experience in party administration, limits his political prospects for the top post," the profile explains.

But on the Afghanistan operation, Ustinov is definitely in the elite inner circle of the Politburo with KGB chief Yuri Andropov, Foreign Minister Andrei Gromyko and candidate-member Boris Ponomatyov. With the KGB boss at his closest ally and the confidence of the military, Ustinov is currently riding high in the Kremlin. This in turn portends a continued ascendancy for the hard-line foreign policy he espouses.