The Hundred Days is indelibly associated with Franklin D. Roosevelt, and the Thousand Days with John F. Kennedy. But as of this week, a thousand days remain of President Bush"s last term -- days filled with ominous preparations for and dark rumors of a preventive war against Iran.

The issue of preventive war as a presidential prerogative is hardly new. In February 1848 Rep. Abraham Lincoln explained his opposition to the Mexican War: "Allow the President to invade a neighboring nation, whenever he shall deem it necessary to repel an invasion and you allow him to do so whenever he may choose to say he deems it necessary for such purpose -- and you allow him to make war at pleasure [emphasis added]. . . . If, today, he should choose to say he thinks it necessary to invade Canada to prevent the British from invading us, how could you stop him? You may say to him, "I see no probability of the British invading us"; but he will say to you, "Be silent; I see it, if you don"t." "

This is precisely how George W. Bush sees his presidential prerogative: Be silent; I see it, if you don"t . However, both Presidents Harry S. Truman and Dwight D. Eisenhower, veterans of the First World War, explicitly ruled out preventive war against Joseph Stalin"s attempt to dominate Europe. And in the Cuban missile crisis of October 1962, President Kennedy, himself a hero of the Second World War, rejected the recommendations of the Joint Chiefs of Staff for a preventive strike against the Soviet Union in Cuba.

It was lucky that JFK was determined to get the missiles out peacefully, because only decades later did we discover that the Soviet forces in Cuba had tactical nuclear weapons and orders to use them to repel a U.S. invasion. This would have meant a nuclear exchange. Instead, JFK used his own thousand days to give the American University speech, a powerful plea to Americans as well as to Russians to reexamine "our own attitude -- as individuals and as a nation -- for our attitude is as essential as theirs." This was followed by the limited test ban treaty. It was compatible with the George Kennan formula -- containment plus deterrence -- that worked effectively to avoid a nuclear clash.

The Cuban missile crisis was not only the most dangerous moment of the Cold War. It was the most dangerous moment in all human history. Never before had two contending powers possessed between them the technical capacity to destroy the planet. Had there been exponents of preventive war in the White House, there probably would have been nuclear war. It is certain that nuclear weapons will be used again. Henry Adams, the most brilliant of American historians, wrote during our Civil War, "Some day science shall have the existence of mankind in its power, and the human race shall commit suicide by blowing up the world."

But our Cold War presidents kept to the Kennan formula of containment plus deterrence, and we won the Cold War without escalating it into a nuclear war. Enter George W. Bush as the great exponent of preventive war. In 2003, owing to the collapse of the Democratic opposition, Bush shifted the base of American foreign policy from containment-deterrence to presidential preventive war: Be silent; I see it, if you don"t. Observers describe Bush as "messianic" in his conviction that he is fulfilling the divine purpose. But, as Lincoln observed in his second inaugural address, "The Almighty has His own purposes."

There stretch ahead for Bush a thousand days of his own. He might use them to start the third Bush war: the Afghan war (justified), the Iraq war (based on fantasy, deception and self-deception), the Iran war (also fantasy, deception and self-deception). There is no more dangerous thing for a democracy than a foreign policy based on presidential preventive war.

Maybe President Bush, who seems a humane man, might be moved by daily sorrows of death and destruction to forgo solo preventive war and return to cooperation with other countries in the interest of collective security. Abraham Lincoln would rejoice.

Arthur Schlesinger Jr. is a historian and served as an adviser to President John F. Kennedy.