An interactive inquiry about why America hasn't been attacked again

The introduction to and eight parts contained in "Why No More 9/11s?" are reprinted here for those who prefer to read the entire series on paper. Click the "print" button at right for a text-only version suitable for printout.

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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. In December 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.

I. The Terrorists-Are-Dumb Theory

The 9/11 attacks were heralded as the harbinger of a new era of foreign-based terrorism in the United States. What if they were really a fluke?

Al-Qaida's successful elimination of the Twin Towers, part of the Pentagon, four jetliners, and nearly 3,000 innocent lives makes the terror group seem, in hindsight, diabolically brilliant. But when you review how close the terrorists came to being exposed by U.S. intelligence, 9/11 doesn't look like an ingenious plan that succeeded because of shrewd planning. It looks like a stupid plan that succeeded through sheer dumb luck.

Consider: