THE Pentagon has taken great pains to appear calm throughout the uproar of the last two weeks' events in the Soviet Union.Though defense officials made "lots of unscheduled trips to Pennsylvania Avenue" and the White House, in the words of one knowledgeable military officer, there was little change in pace for United States armed forces. The coup attempt was discussed at a regular meeting of the Joint Chiefs of Staff, but no special meetings were held. No US military units had their alert status changed, several officials say. Given the size of the Soviet nuclear arsenal, the US national security establishment took care not to do anything that could make their Soviet counterparts nervous. The Soviets apparently felt the same way. "At no point did we see any cause for alarm," says a top US military source. During the coup, Soviet military officials deliberately lowered the profile of their strategic nuclear systems, said Pentagon spokesman Pete Williams last week. The alert status of some systems went to a below-normal level, he said. Some news reports said the head of the Soviet Strategic Rocket Forces ordered SS-25 mobile missiles to be withdrawn from field locations and stored. As the USSR continues to unravel, the Pentagon's biggest concern is that centralized control over nuclear weapons will break down. Though this is seen as unlikely, all possible US intelligence assets, such as spy satellites, have been focused on surveillance of Soviet nuclear sites, a Pentagon officer says. What particularly worries US analysts is the chance that the republics will gain control of nuclear weapons on their territory. The US estimates that 80 percent of Soviet long-range nuclear weapons are based in the Russian Republic. But Kazakhstan and the Ukraine have silo-based ICBMs, and Byelorussia has locations for mobile ICBMs. To this point Kazakhstan and the Ukraine have demonstrated an aversion to the nuclear weapons on their territory. Ukrainian leader Leonid Kravchuk said last week that the nuclear systems in his republic should be removed and agreed to begin talks with Russian President Boris Yeltsin and other republic chiefs about the nuclear question. Kazakh President Nursultan Nazarbayev issued a decree Aug. 29 closing the Soviet nuclear test site at Semipalatinsk. Many eyes in the Pentagon also are focused on the Soviet Union for another reason: The size and shape of the US military is based on the perceived threat of a Soviet counterpart. If the central Soviet government dissolves in a matter of weeks, why will the US need to continue spending close to $300 billion a year on national defense? President Bush said last week that events in the Soviet Union may eventually give the US "an opportunity for a vastly restructured national security posture." Rep. Les Aspin (D) of Wisconsin, chairman of the House Armed Services Committee, has proposed the first step in this direction, saying $1 billion should be shaved from the Pentagon budget and be used to send humanitarian aid to the Soviet Union this winter. Such a move would be "defense by different means," he says. Defense Secretary Richard Cheney, however, said in a tough speech on Aug. 29 that "the euphoria of the moment" shouldn't blind the US to possible national-security dangers ahead. Though real change has occurred in the USSR, it still is unclear who will control the Soviet nuclear arsenal, he said. The breakdown in central authority could lead to the nightmare of battling republics, turning the country into a large, better-armed Yugoslavia. He also echoes other conservative voices who have claimed the events in the Soviet Union have strengthened the case for the Strategic Defense Initiative. Mr. Cheney raises another question: If the Soviet Union ceases to exist as a central authority, what will happen to arms-control agreements? Sites in the Baltics, for instance, would be subject to inspection under the new treaty limiting conventional forces in Europe. If the Baltics become separate countries, what are the implications for the treaty? "Well, we don't know," says an arms-control official. "We're looking at it hard."