ONE of the best reasons for maintaining a limited, temporary Soviet Union is that a union offers time to dismantle and reapportion the huge Soviet military in an orderly way. A dismantling under union conditions is more likely to be conducted by mutual agreement, not nationalistic self-interest, and will provide assurances on international treaties already signed.The Soviet military has been reforming since 1988, when the Kremlin recognized the cost of maintaining 4 million men under arms had become prohibitive. Uncertainties in the wake of the failed coup, and the Army's tacit approval of the coup, mean the process will now be speeded up. The Russian Republic will retain the bulk of the Army and most if not all of the Soviet Navy. If the Soviet center fails to hold, Russia must become responsible for all nuclear weapons. Each republic naturally wants its own army, or "national guard," and each is setting up its own defense ministry. The problem is that dismantling the Soviet military won't happen overnight. The units, particularly armored units, are very large and contain tactical nuclear components built into the command and control structure. These nuclear forces must be separated out - possibly by taking apart whole divisions. Nuclear weapons appear to have been treated sensitively during the coup. A number of commanders ordered their mobile launched nuclear weapons back into garrisons during the crisis. Since last year, the Soviets had been removing tactical nuclear weapons from ethnic hot spots. The removal of nuclear weapons from republics must continue. There is no reason any autonomous republic should have nuclear forces or capacity, and presumably the Russians wouldn't want them to. The terms of the nuclear nonproliferation treaty will be binding on Russia; that means no new nuclear states. Only three republics besides Russia have fixed-silo ICBMs - Kazakhstan, the Ukraine, and Byelorussia. Whether these sites will be dismantled, or set up as Russian "bases such as the US has overseas - is unclear. Soviet submarine missiles fall under current treaties, but the disposition of Soviet surface ships with tactical weapons needs review. As the Soviet center crumbles, the West must affirm existing treaties with the new military leaders, Russian or otherwise.