I imagine there'd be a couple of months of saying how terrible the act was, how it was an atrocious act against not only those who died, but America herself, how that the people who carried out the attack needed to be crushed now before it could happen again... and then you'd invade five or six countries with small ties to said organisation . However, you'd not go it alone. Britain would tag along like the fucking lapdog Blair made us, whilst the rest of Europe would follow Russia and France as they veto their way out of conflict causing the UN to refuse to sanction the war, making it illegal... which wouldn't mean anything or stop anyone.

At least, that's what I think your current administration might do.