The speed with which President Carter accepted French President Valery Giscard d'Estaing's sensible proposal for an all-African peacekeeping force to safeguard Zaire under-scores this fact of life in Washington: National Security Adviser Zbigniew Brzezinski has emerged the clear winner in his foreign-policy struggle with the State Deparytment.

Giscard d'Estating's proposal, along with a requent for U.S. logistical help to get troops and supplies on the scene, was made at a working dinner in the White House May 26. Carter accepted it immediately, without reservations. That abruptly ended 16 months of State Department policy putting Africa off limits to the slightest suggestion of U.S. military involvement.

Simultaneously, the administration ended its headlong rush to a new strategic arms limitation treaty. Apart from the president's rhetoric appeasing the arms control lobby, SALT seems destined for a deep freeze until the November election.

In short, Brzezinski at least for now has convinced the president that the nature of U.S.-Soviet relations must be changed to challenge Moscow's boldly expansionists mood. To losing officials at the State Department, that dooms what they have fought for and nudges Secretary of State Cyrus vance into a secondary role.

Given Jimmy Carter's famed ambiguity, Brzezinski's triumph may be temporary. One middle-level State Department official unhappy with new course told us: "The president wants to be the tough guy and at the same time he wants to be the apostle of peace and arms control. Where does he end up?"

Having asked the question, such officials have pressed a media counterattack against the new policy. Interviewed in U.S. News and World Report this week, U.N. Ambassador Andrew Young when asked about the president's "concern" over the Soviet-Cuban offensive, sniped at Brzezinski. "When concern is translated into panic by others," Young said, "I don't think it serves the U.S. interest."

The principal source of the counterattack against Brzezinski is the Arms COntrol and Disarmament Agency, headed by Paul Warnke. ACDA has been the source of widely scattered reports complaining about the new U.S. coolness in the arms talks.

The president's handling of the counterattack shows his ambivalence. He was displeased with Young's interview. But a few days earlier, he was stung by a June 2 Washington Post headline accusing him of "imposing a freeze" on SALT.

Telephoning Press Secretary Jody Powell at 7 a.m. clearly upset and angry, the president ordered Powell to produce the White House press corps that morning for a personal denial. Instead of accepting the truth of The Post's headline, he took it as a personal insult and claimed that SALT was still enshrined as an immediate administration goal.

Some officials previously favoring a conciliatory policy toward the Kremlin and a hands-off policy in Africa are not counterattacking but accommodating. Critical colleagues refer to State Department policy-planning chief Tony Lake and other important officials as "born-again hawks" who want no more publicity linking them with "softness" in foreign policy.

Between them and the counterattackers stand Cyrus Vance. When Brzezinski returned from his Soviet-baiting trip to China, Vance aides say, he "humiliated" the secretary of state by immediately appearing on "Meet the Press" before even discussing his trip with Vance.

However, if Vance felt humiliated, the record fails to disclose it - possibly because Vance is no theoretician or ideologue but a tightly disciplined lawyer who sees his role as problem-solver under Carter's direct supervision. If the president wants to put SALT in mothballs and wants an activist anti-Soviet policy in Africa, Vance will go along. According to White House aides, he is now more involved in quieting the rebellion against Brzezinski than in pushing views of his own.

That may account for Brzzezinski's policy take-over. He had been silent for months, despite growing alarm at the administration's lack of response to Soviet audacity. Intimates say he hoped that Vance or Defense Secretary harold Brown would change the policy.

Now that he has made his move, motivated by philosophical convictions about the Soviet threat, Brzeziunski intends to keep up the pressure. That contrast with the non-ideological Vance and the silent Brown helps exoplain why and how the professor from Columbia is today responsible for setting the nation's foreign policy.