The idea that the American public could only be pushed into backing for major military action by crashing planes into the World Trade Center is arrant nonsense that spits in the face of history and common sense.
There was actually a rising tide pushing for intervention in Afghanistan for years prior to 9/11 - there was a grass-roots information campaign online outlining the various things the Taliban did on a regular basis (usually tied to anti-women civil rights violations). Back in the days when people actually used to take chain mails more seriously, that was one of the ones that got sent around a lot. And I actually remember seeing protests against the Taliban (along with ones against China over the Falun Gong situation) in NYC years before.
In addition, the only place the US had a vested interest in intervening in at all was Iraq (because oil), and we essentially already HAD a mandate to intervene there if we wanted to due to the lingering issues from the Gulf War, and events like the Cole bombing and the 1998 embassy bombings providing easy pretext (which we explicitly DIDN'T take advantage of). We've started major wars over way less.