It started, as so many things do, with the French. The first 'Hundred Days' of modern times does not involve an American president – not even Barack Obama who is about to complete the most closely scrutinised 'First 100 Days' since Franklin D. Roosevelt set the standard back in 1933, in the depths of the Great Depression.
No, blame the French official who spoke of "Les cent jours" to describe that fleeting but pivotal period of European history between Napoleon's return to power in March 1815, and his final defeat and exile, little more than three months later. Roosevelt's achievement was to turn a simple number into the yardstick by which every American president is now judged.
He did so by summoning the US Congress into a special session on March 9 1933, five days after his inauguration (these things happened then on March 4, not January 20 as today). The session lasted 100 days. By the time it was over in mid-June, 15 major bills had been passed that re-made the country's shattered banking system and laid the foundation for the New Deal, in a frenzy of lawmaking unmatched before or since in America.
Ever since, 100 days has been the traditional first moment of judgement for a president's achievement – though naturally the concept is trumpeted more by Democrats than Republicans, less keen on activist government.
In one sense, the time frame is totally arbitrary and absurd. No special event will mark its passing; life will be pretty much the same on April 30, Obama's Day 101. Yes, that epic session of Congress in 1933 lasted 100 days: but why is a president not judged on his performance over his first 50, 200 or 500 days – or even seven? After all, that was all God needed to create the world. And what happens in those first three-and-a-bit months is rarely an exact pointer to history's verdict on the success or failure of a presidency.
Take Gerald Ford. His decision in September 1974, exactly a month after taking office, to pardon his reviled predecessor Richard Nixon is widely seen as having sealed his defeat by Jimmy Carter two years later. Americans were furious; many believed the whole episode was a fix: the presidency for Ford in return for the guarantee for Nixon that he would not face criminal prosecution. Today, however, almost every historian believes Ford did the right thing.
Bill Clinton is another example. His first 100 days were, not to put too fine a point on it, a disaster. Day One was consumed with an embarrassing flap that led to the withdrawal of the nomination of Zoe Baird to be America's first female attorney general (that distinction would ultimately go to Janet Reno), and the following 99 were an unfolding proof of Murphy's Law.
Next came a needless dispute over gays in the military that seemed to brand Clinton a feckless liberal, and the controversial appointment of his wife to head a secret health care reform task force. There was also much discussion of the new commander-in-chief's flabby salute, hardening a suspicion that the former Vietnam draft-avoider retained a scorn for the armed services. Nor was Clinton praised for his handling of the Waco siege. Rounding off the 100 days was a fuss over the $200 haircut he received on Air Force One as it was parked on the tarmac at Los Angeles Airport. Reports that all air traffic was halted for hours over southern California as a result of the presidential beautification were unfounded. But the public perception of Clinton as a spoiled child of the 1960s was set in stone. But those fraught cent jours are now a forgotten prologue to what is generally seen as a successful presidency – at least in the light of what came next.
And yet this utterly artificial period reflects a truth about democratically-elected governments. The authority of a president is never higher than in his first months. He (or she) is a new face, with a direct popular mandate conferred by election victory. This honeymoon may last less or more than 100 days. But while it lasts, a leader has rare clout, even when the legislature is controlled by the opposite party.
In 1981 Ronald Reagan proved the point by driving his programme of massive tax cuts – the deed that launched a quarter-century of Republican dominance – through a House of Representatives in which Democrats held a 50-seat majority. And of course, for a while at least, an incoming president can blame everything on his predecessor (which Obama is doing right now).
Other 100 days have attracted less fanfare. Lyndon Johnson had already been president for almost a year when he won an election in his own right in November 1964. But he was acutely conscious of the opportunity it gave to launch the 'Great Society', the most ambitious programme of federal reform since the New Deal. "Jerk out every damn little bill you can," LBJ told his Congressional liaison aides, intent on a debut to match that of FDR.
Neither Bush made waves during his first months. The father talked vacuously about a "kinder and gentler" America, while the son confined himself to partisan gestures on social issues. Indeed the presidency of the younger Bush truly started on Day 235 – September 11, 2001.
Even JFK, whose arrival as a young Democrat prefigured that of Obama, was restrained. Before the election, he had talked of bold legislative initiatives. But his narrow win over Richard Nixon made him proceed cautiously. In the event, the signature incident of JFK's first 100 days was the Bay of Pigs invasion of Cuba.
And perhaps the most consequential 100 days of all attracted no fanfare whatsoever. For Harry Truman, thrown in at the deep end by Roosevelt's death on April 12, 1945, the mere mention then of his own 100 days would have been in the poorest of taste. In the course of them, however, the untested Truman had to cope with the end of the war in Europe, the critical Potsdam conference with Stalin, FDR's death, and the successful testing of the first atomic bomb (of whose very existence he knew nothing until he was thrust into the White House). Truman's 100 days ended, more or less, with his decision to use the terrible new weapon against Japan.
For all the foreign policy issues confronting Obama, none (yet) poses as momentous a choice. Otherwise, the fixation with the first 100 days has for once been justified. Like FDR, Obama came to office facing an overwhelming economic crisis that demanded immediate action. Just as in 1933, the country is at a political and ideological watershed. The quarter-century when conservative thinking held sway is over. Unfettered markets have failed the country. For Reagan, government was the problem. Today, Americans look desperately to their government for answers – and those answers may add up to an American version of social democracy.
If anything, Obama is being criticised for doing too much, too fast. Bank bailouts and gigantic stimulus packages are only part of the story. On the foreign front he has moved to 'reset' relations with Russia, Iran and Cuba. He is reversing Bush policies on conservation, green energy and climate change. He has already taken action that will save (or doom) the US car industry. He has announced plans for a high-speed rail system. He is about to wade into the swamp of health care reform with the bold – perhaps impossible – goal of universal coverage.
The hyper-activism is deliberate. All the balls are up in the air, offering opportunities for change that a few years ago would hav been unthinkable. As Obama's chief of staff Rahm Emanuel puts it, "you never want a serious crisis to go to waste."