Wars do things you don't want them to do.

Matthew Yglesias on "The Mostly Hypothetical Case for Armed Humanitarianism:"

"I think it’s telling that enthusiasts for this kind of war typically have to make the case with reference to hypothetical success stories about military operations we didn’t undertake. These are useful cases to deploy in arguments, because since the intervention didn’t happen one doesn't need to wrestle with the potentially problematic consequences and downside risks. [...] I think it’s a problem when all your best evidence is drawn from scenarios that didn’t unfold." (emphasis mine)

One reason to oppose war, to avoid war, and to be skeptical of claims calling for war, is that wars almost always have unpredictable, unforeseen consequences. These unexpected consequences, which can be deeper, wider, and go on longer than war proponents' imaginations seem to grasp, are almost always negative. When you unleash a war, that war becomes a thing itself, going places and doing things you didn't know it would do and certainly didn't plan for.