The obvious point, though not obvious enough to mute the “100 Days” frenzy, is that events will often alter the core assumptions of a president. Kennedy came to office with speeches that bristled with Cold War assumptions about the domino theory and the need for America to design new tools to take the offensive in guerrilla warfare. After coming perilously close to nuclear annihilation during the Cuban Missile Crisis, his premises changed radically. By the time of his assassination, he was embarked on an effort to tamp down the Cold War and find common ground with the Soviet Union. Compare his American University speech of June 1963 with his 1960 campaign rhetoric, and it’s hard to believe it’s the same man talking.

George W. Bush came to office calling for “humility” in foreign policy, and warning against the overreach of nation-building. In the wake of the September 11 attacks on New York and Washington, Bush migrated to a policy that promised a wholesale flowering of democracy throughout the Middle East; by his 2005 Inaugural, he was proclaiming that “it is the policy of the United States to seek and support the growth of democratic movements and institutions in every nation and culture, with the ultimate goal of ending tyranny in our world”—a policy that outdid Woodrow Wilson in its promise and that proved just as illusory as Wilson’s had been almost a century earlier.

The list of the utterly unpredictable is stunning. Did Nixon’s first 100 days offer a clue that he would be toasting the health of Mao Zedong in Beijing a few years later? That Ronald Reagan would be strolling through America with the leader of the Soviet Union? That Barack Obama, who led his party to a smashing triumph in 2008, would preside over the implosion of the party at every level over the next eight years?