Whenever someone asks me that, I just say that he violates numerous UN resolutions, and that the 2003 war was a resumption of hostilities because he violated the cease-fire treaty from Gulf War I. Pretty simple.
The fact that Hussein's military was firing at U.S. and British patrol craft WEEKLY the whole decade from 1992 through 2003 is enough for me to have supported going to war in Iraq. Last time I checked, firing at a nations planes was an act of war when it happened one time, one day. Hussein had his military do that hundreds of times over the course of 11 years, and we were authorized by the U.N. to fly there to protect civilians from attacks by Hussein. We should have gone in and removed Hussein by force in the early 90s the first time he fired on one of our planes patroling the no fly zone. But that would have required Clinton taking his testicles back from his wife so of course it never happened.
....AND positively the truth.