Events in Moscow and Washington within the past 24 hours provide a vivid snapshot of all the fears that drive the nuclear arms race in the United States and the Soviet Union, and the action-reaction cycle that seems to keep it out of control.

On Wednesday, Soviet President Leonid I. Brezhnev delivered a sternly worded speech to his top military leaders, promising "measures to meet all your needs" in response to what he called an "unprecedented" arms buildup by the Reagan administration.

In response, Defense Secretary Caspar W. Weinberger told a Pentagon news conference yesterday that Brezhnev's "challenge emphasizesthe correctness . . . , as nothing else could," of not flinching from the $1.6 trillion five-year military buildup laid out by President Reagan, who says Moscow already enjoys military advantages over this country.

Brezhnev stressed to his marshals and ministers that they must not fall behind in the technological race for new weapons. His remarks, and other official statements recently, hinted at concern that the Soviets traditionally have been behind the United States in weapons technology and are still behind.

At the Pentagon, however, Weinberger pulled out charts to show how old American missiles, bombers and submarines are compared with their Soviet counterparts. His message was that it is the United States that is now behind. Bombers must be modernized to get through new Soviet air defenses, and U.S. land-based missiles must be made less vulnerable to attack by newer Soviet missiles, he said.

Brezhnev charged that it was the Reagan administration's "aggressive policy that is threatening to push the world into the flames of a nuclear war." The Soviets have frequently cited talk in Washington about fighting and prevailing in a protracted nuclear war to bolster their claim that the Reagan administration believes a nuclear war can be won.

But Weinberger said he sees increasingly in Soviet literature that the Soviets "think nuclear war is winnable."

Undoubtedly, the tough talk by Brezhnev and Weinberger is partly verbal posturing as both sides approach crucial and expensive decisions on deployment of more weapons, which in turn, will produce moments of truth in the arms negotiations now underway in Geneva.

At the same time, there is a high-stakes battle in this country and Western Europe for public opinion to support addition of new arms and big military budgets. But the tough talk also brings into sharp focus how much momentum there is in the official hard-line position in both countries, how hard it is to overcome that, and how vast is the gap in perceptions of each other's power.

In his speech, Brezhnev appeared to place special emphasis on the importance of the race for technological supremacy in weapons, and this may prove to be the most important stumbling block in eventually coming to arms control agreements or easing concerns of the Soviet military.

For example, Soviet weapons were badly outclassed by U.S.-made weapons in the recent fighting in Lebanon; even though the Soviet weapons were not their best and were used by Syrians, American analysts believe the results were a jolt for the Soviets.

Furthermore, the U.S. analysts say Moscow appears very concerned over the American technological and production lead in jet-powered cruise missiles.

Thousands of these new nuclear-tipped weapons are to be deployed soon and Moscow is particularly anxious to limit that deployment, especially on submarines and ships, in the Geneva talks.

"Competition in military technology has sharply intensified," Brezhnev told the military commanders, "acquiring a fundamentally new character. A lag in this competition is inadmissible.

"We expect that our scientists, designers, engineers and technicians will do everything possible to resolve successfully all tasks connected with this."

It was the second time this year that this theme surfaced publicly. In March, Marshal Nikolai V. Ogarkov, chief of staff of the Soviet armed forces, warned in a new book of "serious consequences" of any letup or "stagnation" of attitudes toward new weapons development.

In a remarkably candid commentary, Ogarkov noted that the United States was first to make the crucial breakthroughs in weaponry for the last several decades. He cited the atomic and hydrogen bombs, nuclear-powered submarines, multiple warheads for missiles, aircraft carriers, airborne warning and control system (AWACS) planes and equipment, lasers, stabilized tank turrets and the space shuttle, among other things.

In reviewing Brezhnev's speech yesterday, some senior American analysts said they thought it was meant mostly as reassurance to the Soviet brass that they would continue to get all the resources they need to keep pace or catch up. That seemed to be Weinberger's message as well.