The horror of 9/11, to be relived this week in memorials and speeches, remains for most Americans the worst disaster they have ever witnessed, the most unnerving bit of chaos ever to intrude upon their world.
But it could have been much, much worse.
Experts on the Twin Towers watching their collapse guessed at 20, 30, or even 50 thousand dead.
The inability of one terrorist pilot to see the White House probably kept that building from being hit. The delay in take-off of Flight 93 and the heroism of its passengers prevented the Capitol from being destroyed.
The deep partisan division that has split the country in the half-decade since 9/11 may find its origins not just in specific issues such as the Patriot Act but also and perhaps more profoundly in the different understandings people have of the scale of disaster that 9/11 represents.
Those who believe that those 19 terrorists did the worst that terrorists could ever realistically do to the country might believe that one lucky attack-one piercing of the homeland defense-does not justify the forward defense involved in the invasions of Afghanistan or Iraq. They see the various surveillance programs as overkill, and the London arrests as evidence not of the need for continued and increased vigilance, but of the effectiveness of in-place programs.
Those who think that 9/11 could have been worse, and that the terrorists plan other attacks, recoil from the prospect of letting down our collective guard or decreasing the sense of urgency.
The urgency in the president's, the vice president's, and the secretary of defense's speeches over the past fortnight has been dismissed by their political opponents as fear-mongering, cynical posturing in the run-up to the November elections.
President Bush strikes his supporters as committed to completing his tenure without another devastating attack, one perhaps exponentially more deadly than the last. They understand his addresses to be the plain-spoken warnings of a leader who wakes up every day to a briefing about terrorist plotting around the globe.
President Bush's most powerful argument: the words of our enemies. Americans who believe the worst of the president and his administration have to believe as well that our enemies are either blowhards or incompetents. But Americans who take seriously those threats hope they will never be unarguably proven right.