Amid the many uncertainties loosed by the al-Qaida attacks on Sept. 11, 2001, one forecast seemed beyond doubt: Islamist terrorists would strike the United States again—and soon. "Ninety days at the most," said counterterrorism expert Juval Aviv. On Oct. 5, 2001, an unnamed senior intelligence official told Congress, in a private briefing, that there was a "100 percent" chance of another terrorist attack should the U.S. invade Afghanistan, as it did two days later. "An attack is predictable now whether we retaliate against Afghanistan or not," reasoned House Speaker-to-be Nancy Pelosi, D-Calif. Sen. Richard Shelby, R-Ala., agreed: "You can just about bet on it."
When no second terrorist attack occurred in 2001, experts adjusted their time horizons. "If we get through the summer without some sort of attack, we'll be pretty fortunate," said George Vinson, a security adviser to then-California Gov. Gray Davis, in June 2002. In February 2003, Tom Ridge, the nation's first secretary of homeland defense, publicly estimated an 80 percent likelihood that terrorists would attack the United States within the next few days. In August 2003, the World Markets Research Center said it was "highly likely" that terrorists would attack the United States within the next 12 months. In June 2006, unnamed U.S. officials told CBS News they'd be surprised if the United States weren't hit by a terrorist attack by the end of that year. InDecember 2008, the Commission on the Prevention of WMD Proliferation and Terrorism said it was "more likely than not" that by the end of 2013, terrorists would attack somewhere in the world using a chemical, biological, or nuclear weapon. In a Feb. 4 interview with Politico, former Vice President Dick Cheney said there was "a high probability of such an attempt." He didn't say when.
It didn't happen—or, rather, it hasn't happened yet. Islamist terrorists struck Bali, Madrid, London, Mumbai, and many places in and around the Mideast, but they haven't struck the United States. Why not? The question is impossible to answer with certainty. But given that the "war on terrorism" was (for good or ill) the defining pursuit of George W. Bush's presidency, anyone seeking to understand the previous eight years of American political history must ask it. More urgently, our new president, Barack Obama, is surely pondering this question as he assesses the present risk of a terrorist attack on the United States and how best to address it.
I spent the Obama transition asking various terrorism experts why the dire predictions of a 9/11 sequel proved untrue and reviewing the literature on this question. The answers boiled down to eight prevailing theories whose implications range from fairly reassuring to deeply worrying. To explore them, click on the worry beads in the illustration above. If you prefer instead to print out this introduction together with all eight parts of the series, click here.