I saw a bumper sticker once that said "I'm even against the next war," and that sort of sums me up (I used the same line a few posts back, and use it fairly often in these discussions).
I don't trust our leaders and the people who make these decisions. Obviously, there are times when war is necessary - WWII is the classic example. We were attacked, and the entire world - including our allies - were under immediate threat from tyranny.
Most wars since just haven't passed that test, and many haven't even come close. I feel like 2003 was a situation where we really took the concept of war lightly. Cheney was talking about 6 months, and no one was considering the long-term ramifications of invading. If history has shown us anything about war, it's that war really opens up a Pandora's Box of possibilities, and can lead to a lot of outcomes that no one saw coming. If we had known that the Iraq War would be a 10+ year commitment in '03, no one would have gone for it.
A long-winded explanation, but I think it's an important question. The answer, for me, is that you only engage in war when it is an absolute last resort, and all other options have been exhausted.