There's a good piece by Jonathan Raban in the Independent on what really changed after 9/11.
"Since September 11..." we say, as if the attacks were what changed everything. The month is right but the day wrong, because the real metamorphosis has arisen not so much from what Mohamed Atta and his co-conspirators did to us on September 11 as what we've subsequently done to ourselves - and continue to do, today, tomorrow, and in the foreseeable future (incredibly foreshortened though that has become). On September 12, still in shock at the extraordinary injury inflicted on the US, we woke to essentially the same world we'd been living in before the phones began to ring. The death toll - then estimated at 10,000-plus - was horrifying, on the scale of a major earthquake or tsunami, but the globe continued to revolve on its accustomed axis, as it does after even the most devastating seismic killers. ...
Not 9/11, he argues, but 9/18 is "the real date to circle."
That day, Congress rushed through its Authorisation For Use of Military Force (AUMF), entitling the President, as the nation's commander in chief, to "use all necessary and appropriate force" against "those nations, organisations, or persons" that "he determines" were responsible for the September 11 atrocities, "...in order to prevent any future acts of international terrorism against the United States by such nations, organisations, or persons." It's the "such" that's the key, the inclusion of nations, organisations, or persons "of that sort", which nicely covers, for instance, the invasion of Iraq, the arrest and detention of most of the prisoners now languishing in Guantanamo Bay, possible future military action against Iran, or Syria, or both, and heaven knows what else, since "such" is a term of potentially limitless capacity to make hitherto unguessed-at likenesses and connections.
The sloppily-worded AUMF endowed the administration with unique and wide-ranging powers. It has become the licence for the executive branch to wave at Congress and the judiciary whenever its actions are questioned or censured. On September 18 2001, the delicate balance between the three branches of government, as laid out in the American constitution, was thrown severely out of whack; since that day, one branch, the presidency, has enjoyed an unprecedented primacy over the others, and we've been living with the consequences of AUMF ever since.
Worth reading in full. Also worth a look is this interview with Raban about his book, My Holy War.