LONDON -- Could the Cold War be about to begin all over again? That is the gloomy question being asked by a number of defense analysts and gurus as they contemplate a possible decision this summer by outgoing U.S. President Bill Clinton to give the go-ahead for a new National Missile Defense system for the United States.

The pessimists, especially in Russia, argue that this will destabilize everything. At a stroke, they contend, the nuclear balance will be overturned and the entire philosophy on which both the Antiballistic Missile Treaty and the current program of mutual missile disarmament between Russia and the U.S. are based will be invalidated.

Why? Because, goes the argument, an effective antimissile umbrella over the whole North American continent will make the present generation of Russian missiles useless. Mutual deterrence will be at an end. The Russians, it is asserted, will then somehow have to go ahead with a new round of missile building that can defeat NMD and remain a threat.