I started out supporting the Iraq war. In the last year or so I've come to myself and decided that it was a bad idea. Of course I've decided that American interventionism in general is a bad idea.
Here are two articles from the excellent blog Notes From A Common-Place Book which very well explain a "conservative opposition to the war. I don't entirely agree with everything written therein, but it is a good start:
One from the LA Times
The other from the normally idiotic New York Review of Books
Read and be edified.