I've always been a bit iffy about OIF, although that's the campaign I fought in, rather than OEF (the one that I feel is completely legitimate).

However, evidence I can remember off the top of my head supporting the legitimacy of OIF is thus: Saddam staged it to look like he was in possession of WMD's, due to the increased tensions between Iraq and Iran. Since he didn't want to get into war with Iran, he faked having WMD's as a deterrent, admitting that it was a gamble as he didn't know whether this would provoke the west. Well, his Arch Nemesis' son got elected, and we know what happened there. I beleive Saddam admits all of this in the interview shown on 60 minutes about 6 years ago, a few months before his execution. Unless I'm mistaken, this is the video. Of course, on top of that he was as cruel a dictator as Stalin, so one could argue that the greater good was served in bringing him to justice, though I abstain from giving an opinion there.

I will, however, state that our war against the Al Qaeda is completely legit. Back during the Russian-Afghan War, our Special Forces trained and armed the Islamic Unity of Afghanistan Mujahideen (in order to fight the Russians, since anything we could do to weaken the USSR gave us an advantage in the Cold War), which included Mujahideens that would eventually turn into rogue cells (AKA terrorist groups), like Osama bin Laden's Al Qaeda. We trained this dog to kill, it got rabid, and it's our job to put it down.

As far as other terrorist organizations or a War on Terrorism in general, it's complete bullshit. It's not our job to single-handedly try to clean up the world. If it's not an international effort, then we should only concentrate on cleaning up our own messes.

Corollary: I am merely expressing my views as an American Citizen with first hand experience of the conflict, my views do not represent those of the Army, Department of Defense, US Gov't, et cetera. Additionally, I am writing my final research paper for my composition class on why this war is justified due to the fact that it's us cleaning up the shady shit we did under the Truman Doctrine during the Cold War, so if I come across any contradictory evidence in my research, I will update this post to reflect the more accurate account.

EDIT:

TL;DR

OIF: Saddam admitted to making it look like he had nukes to dissuade war with Iran, thinking US wouldn't give two shits. Then Bush was elected.

OEF: The US Special Forces ended up empowering groups like the Al Qaeda during the russian-Afghan War. Our pets went rogue, it's our responsibility to clean up our mess and put down the monsters we made.

EDIT 2: proofreading