Long before the atomic bomb turned night into day in the desert of Alamogordo in July 1945, it was an idea in the minds of scientists, who deeply pondered the political and moral dilemma they were about to impose on the world. With few exceptions, they arrived at a basic conclusion. The great physicist Niels Bohr articulated it well when he said, "We are in a completely new situation that cannot be resolved by war." The reasons were clear and inescapable. In the first place, thanks to the unlimited destructive power of nuclear weapons, nuclear war "cannot be won and must never be fought," as Ronald Reagan was to put it much later. In the second place, the knowledge on which the bomb was based was destined, like all knowledge, to spread. In the long run, there would be no "secret" of the bomb. The conclusion was equally clear: If nuclear danger was to be contained or lifted, the task had to be accomplished by political means–above all, by international agreements.

The first and most ambitious of these–the Baruch plan, which was put forward by President Truman and called for the abolition of nuclear weapons–was rejected by the Soviet Union, which then put forward a plan that was rejected by the United States. The arms race that the scientists had hoped to head off began. Nevertheless, for the rest of the century the world followed the scientists' advice: Except on one occasion, no nuclear power used force to stop another power from getting nuclear weapons. The pattern was set in the late 1940s, when the United States declined to launch a pre-emptive attack on the Soviet Union in the years before it got the bomb. In the early days of the Soviet nuclear buildup, President Eisenhower likewise rejected what he called "preventive war." "How could you have one," he said at a press conference, "if one of its features would be several cities lying in ruins, several cities where many, many thousands of people would be dead and injured and mangled?" The pattern held when China launched its nuclear weapons program: Neither the United States nor the Soviet Union launched a pre-emptive attack. The one exception was the Israeli attack in June 1981 on a reactor that Iraq was using in its nuclear-weapons program.

All other attempts to stop the spread of nuclear weapons or reduce existing arsenals have been diplomatic and political. They include the Nuclear Nonproliferation Treaty, which came into force in 1974, the SALT and START treaties, under which the nuclear arsenals of the United States and the Soviet Union, and then Russia, have been cut by half, the Comprehensive Test Ban Treaty and the Anti-Ballistic Missile Treaty of 1972.

In his State of the Union address on January 29, George W. Bush, in one of the sharpest and most significant policy shifts of the nuclear age, overthrew this consensus of more than a half-century. He announced his decision to do just the thing that Niels Bohr said was impossible: to try to solve the nuclear dilemma by waging war. His words left no room for doubt about his intentions. After lumping together Iraq, Iran and North Korea with the odd locution "axis of evil," he said, "I will not wait on events while dangers gather. I will not stand by as peril draws closer and closer. The United States of America will not permit the world's most dangerous regimes to threaten us with the world's most destructive weapons."

The historic importance of the shift was concealed by the context in which Bush placed it, namely the "war on terrorism." A radically new policy was presented as a mere expansion of an existing one. The segue came when he said that the evil-axis nations "could provide these arms to terrorists, giving them the means to match their hatred." After the fall of the Taliban, much ink was spilled speculating on what "phase two" of the war on terrorism might be. Would the United States chase Al Qaeda into Indonesia, Pakistan, Somalia, Lebanon? Now it turns out that phase two is not a war on terrorism at all but a whole series of much larger wars to stop the spread of weapons of mass destruction–history's first disarmament wars.

The new nonproliferation strategy is in truth only the culminating move in a much broader shift in American policy from diplomacy to force–or to put it more plainly, from peace to war. An accompanying move has been the widely uncommented-upon US exit from the entire structure of nuclear arms control treaties that were built over the past thirty years or so. In 1999 the Senate refused to ratify the test ban treaty. Late last year, the Bush Administration gave notice that its continuing reduction of strategic nuclear arms would occur outside any treaty, putting an end to START. A few weeks later, the Administration announced its withdrawal from the Anti-Ballistic Missile treaty, the better to build national missile defense. Only the Nuclear Nonproliferation Treaty, to which the United States belongs as a nuclear power, remains intact, and it has never managed to put any constraints on US behavior. Its raison d'être in the eyes of the United States has always been to constrain not the United States but the 182 nations that have agreed to forgo nuclear weapons. In any case, the new Bush policy clearly announces that the true prevention of proliferation is not to be any treaty but American attack.

These policies form a unity: The United States, safe behind its missile shield, will, at its sole discretion and unconstrained by treaties or even consultation with allies (there was no real consultation with the NATO countries on the new policy and no mention of NATO in Bush's address), protect its territory and impose its will in the world by using its unmatched military power to coerce or destroy, if possible by pre-emptive attack, every challenger.

Nothing Bush proposes, however, has undone the elementary truths that led Niels Bohr to warn, years ago, against trying to solve the nuclear dilemma by war. The ABM treaty can be torn up, but the laws of physics cannot. Smart bombs can destroy armies, but not even the most brilliant of them can remove a thought from a person's mind, or stop its conveyance to the mind of another. These are lessons that the world learned, however imperfectly, at the dawn of the nuclear age and that have been confirmed by more than a half-century of experience since. How many wars will be fought and how many lives will be lost before we learn them again?