Most of the anti-war people I speak to don't have a clue why we went to war in Iraq, so they fallback on the familiar  cliches: Bush lied, Cheney and Halliburton, etc. I've condensed my reasons for going to war argument to this:

1. Before 9/11 the Middle-East nations supported terrorism, or were indifferent to it.
2. Until 9/11 the USA did little about terrorism and whatever it did do was defensive in nature.
3. The Administration decided after 9/11 that it was in the nation's best interests to go on the offensive against terrorism.
4. There were two possible strategic targets: the terrorist organizations which were decentralized and elusive,  and the host/supporter regimes. The Administration chose to take action against what for many reasons they judged to be their best and most productive target: Iraq.
   A. Iraq had already been identified by the world community as a rogue state and actively supported terrorist groups.
   B. Iran was a larger threat but a more powerful and costly target.
   C. Syria was a client of Iran.
5. By invading Iraq the US would force a regime change,  give notice to the Middle-Eastern nations that  the US would take military action against those states that supported terror organizations, and place a "big stick," as Teddy Roosevelt called it, right in the middle of the region.

This explanation of the reasons why I support our war effort always elicits the same reaction: I never heard that before, or something similar.