FOR THE coming NATO summit, President Bush has designed a nuclear centerpiece -- a reformulation of the terms on which the United States would be prepared to use tactical nuclear weapons in the defense of Europe. You could say that the changes flowing from the Soviet Union's retreat from Eastern Europe and from Soviet and Western decisions to reduce their immense military forces across the old Iron Curtain have already done more for the defense of Europe than any Bush statement. "Life itself," as Mikhail Gorbachev is given to say, is transforming the East-West nuclear confrontation, rendering almost unimaginable the possible considered state use of nuclear weapons and the onset of war on the continent. Still, it befits a great power to have a strategically rational and politically sustainable concept of how to handle nuclear arms, and this is what Mr. Bush is trying to do.

Under the old formula, Washington had declared that if the Soviets, through their great advantage in conventional forces, invaded and threatened to break through, the United States would consider responding with nuclear weapons -- the when and how were always appropriately vague. The United States rejected calls to forswear "first use" of these weapons, believing that to do so would sentence the allies to live under the fear of Kremlin conventional assault. This logic, which was called "flexible response," satisfied most strategists and most Western publics for decades. It did its part in keeping the peace.

Now Washington is moving to address Moscow's decision to abandon its East European empire, its huge conventional edge and its invasion and intimidation capabilities. As the specter of a Soviet juggernaut vanishes, so grows a requirement to fit nuclear weapons politically and militarily to new circumstances. Hence the new depiction of American Europe-based nuclear arms as "weapons of last resort."

Nuclear strategy always and necessarily combines instructions to the military, signals to the adversary and assurances to the public. The new words do not necessarily change instructions to the military. But they signal attentiveness and calm to a Europe of which the Soviet Union is increasingly a part; meanwhile, the United States prepares to draw down short-range nuclear artillery.

Thus reassured, Europeans and Soviets should be readier to permit NATO the political space in which to decide in good time -- as conventional force reductions proceed and as a new security "architecture" takes shape -- what nuclear configuration best suits the emerging East-West world.