Presuming that the destruction on 9/11/2001 was, in fact an attack by terrorists outside the US, and presuming further that those terrorists were -as seems quite widely to be held as de facto truth- carrying out the attack on behalf of al Qaeda, a military response against al Qaeda seems justified. Since al Qaeda is not a nation, any reasonable military response must be expected to be deployed into whatever nations and regions in which al Qaeda is operating in order to strike out against this -presumed- imminent threat. Which we did, for a few months (the most important of al Qaeda were out of our sights (and out of our minds?) within about 9 weeks), except that when al Qaeda fled into the lands of our ally, we did not follow them. After March of 2002, as far as I can tell, the only thing the US pro-actively did with regard to al Qaeda was to shut down the department within the CIA which was concerned with hunting its leader. ((But we stayed in Afghanistan, fighting the Taliban and other rebel groups unrelated to al Qaeda and 9/11.))
Years later, after we’d kicked out their secular leaders and before Iraq began to govern itself as a theocracy (read: during the military occupation of Iraq, from April, 2003 to June 28, 2004), al Qaeda and a separate Jordanian terrorist organization took interest in Iraq, and several months after the sovereign Iraqi Interim Government was in charge of Iraq, the two terrorist groups officially merged and the new Jordanian branch of al Qaeda began to move into and take action in Iraq as its new democratic government was trying to form. Due to insufficient and inadequate actions and control by American/coalition forces in both regions, al Qaeda now has a strong presence in both Pakistan and Iraq. Finally, and only by creating a vacuum of power and by destroying any semblance of security in Iraq, al Qaeda is present there, and now, -presuming al Qaeda was involved in the 9/11/2001 attacks- going after al Qaeda in Iraq -presuming we have the new Iraqi government’s consent to do so- is starting to make sense.
But please. Don’t try to tell me that there’s a straight line from the WTC to Iraq. The “al Qaeda” we’re fighting there weren’t even al Qaeda until over three years after the attacks; before we invaded Iraq they were “Jama’at al-Tawhid wal-Jihad” and they didn’t even like al Qaeda. The al Qaeda that is presumed to have been behind the destruction of the WTC is pretty safely hanging out in the already-nuclear-weaponized country of Pakistan. You may have heard of Pakistan recently in the news, on account of the increasingly-out-of-control terrorist groups they harbor have started striking out against their neighbor India. Or not. Maybe you like propaganda better than facts.