You go to war not because you want to, you go to war because you have to.
In 1933 the United States did not have dominant armed forces, but let us cast ourselves back to that time, and imagine instead a world where the United States has a defense superiority similar to the one it has today.
Now, Hitler comes to power in Germany, and begins to build the armed forces which he intends to use to conquer Europe. Some politicians in the United States might advocate the preemptive use of force to effect regime change in Germany, but others would reject this notion, saying, "You go to war not because you want to, you go to war because you have to." Clearly, Germany in 1933 did not pose an imminent threat to the security of the United States, so the United States did not then "have to" go to war against Germany.
We will never know how much destruction, suffering, and sacrifice of life might have been prevented by the timely use of force against Germany when it was ruled by a government led by the National Socialist German Worker's Party.
Iraq in 2000 was not Germany of 1933, the Iraqi Baath party is not NSDAP, and Saddam Hussein is not Adolf Hitler, and the recent war may or may not have been wise, but some of the arguments used against it seem remarkably naive.
Which is more important, whether a threat is imminent, or whether that threat is inevitable? Is it easier to deal with a threat when it is incipient or when it is imminent? Is it always wiser to wait to go to war until "you have to"?