AS the extraordinary metamorphosis of the former Soviet Union unfolds, official Washington is on pins and needles over the future of the Soviet military, particularly its nuclear arsenal. Thus far, senior United States policymakers say, nothing untoward has happened with the thousands of strategic and tactical nuclear weapons that are housed in the three Slavic republics that formed a "commonwealth" on Dec. 8, or in the Central Asian Republic of Kazakhstan.Overall, the military situation appears to be holding, despite the absence of a clear central authority. But "we are very concerned about it," says a senior Bush administration official. The official noted the possibility of civil war, a breakdown of military discipline, and hunger among the troops. Another military coup also cannot be ruled out, the official says. If one looks at a general, for example, who has spent his whole life defending the Soviet Union, "there's bound to be a certain unease," says the official. "The military is the wild card in all of this," Robert Strauss, US ambassador to the Soviet Union, said in Washington yesterday. Foreign Minister "Shevardnadze is more concerned about it than anything else." Speaking of the overall situation in Moscow, Strauss said, "there's an anger that I haven't seen before in the last 10 days to two weeks." He described lines getting increasingly long for bread with prices due to be freed soon. So far, the new "Commonwealth of Independent States formed Dec. 8 by the leaders of the Russian Federation, the Ukraine, and Byelorussia - has sought to address Western concerns about stability. The US responded Dec. 9 with approval that the new confederation has pledged to act in ways that coincide with guidelines laid out by the Bush administration for proper behavior of Soviet republics, including adherence to treaties signed by the Soviet Union, respect for the rights of minorities, respect for current borders, and adherence to democratic principles and the rule of law. "Our chief concern," says State Department spokeswoman Margaret Tutwiler, "is that nuclear weapons remain under safe and secure control, and that nuclear weapons be eliminated in accordance with the START treaty [Strategic Arms Reduction Treaty] and the nuclear initiatives of President Bush and President [Mikhail] Gorbachev." THE leaders of the new Slavic commonwealth stated their intention to keep their nuclear weapons under a unified command, but it remained unclear how that would relate to the current Soviet command of nuclear weapons. On Dec. 14, Secretary of State James Baker III heads for Moscow, Kiev, and Minsk, the capital of Byelorussia and the planned capital of the new commonwealth. He may go to other republics as well, says Ms. Tutwiler. Another area of concern related to nuclear proliferation over which the former Soviet Union's new leaders have little if any control is a feared "brain drain" of the nation's top nuclear-weapons experts. With their employment prospects inside the ex-USSR cloudy, US officials are concerned they may seek lucrative positions in unreliable countries such as Libya, Iran, and Iraq. On the diplomatic front, Bush administration officials are expressing optimism that the US is well-positioned to maintain relations with whatever entity or entities emerge for the collapsing union. Therefore, said White House spokesman Marlin Fitzwater on Dec. 9, "We believe we're in a good position to work with whatever system emerges under the commonwealth, or whatever other system develops." He added that it is too early to talk about US diplomatic recognition of the new commonwealth.