1. As bad a guy as Saddam Hussein was, he would have cleaned ISIS's clocks by now. Sometimes leaving bad enough alone really is the best solution (especially if there really aren't any weapons of mass destruction).
2. It's not that long ago that John McCain was blasting President Obama for not helping the Syrian opposition, which mainly consisted of ISIS. Now he's blasting President Obama for not going to war against ISIS. Sometimes, leaving bad enough alone really is the best solution (especially if the choice really is between Bad Guy A and Bad Guy B).
3. Of course, "the best solution" doesn't imply that there actually is a really good solution. Sometimes there isn't.
4. The older I get, the more convinced I become that the overwhelming majority of people in the world just want to quietly live their lives in peace and security, and would happily do so if they didn't keep getting stuck in the middle of one band of thugs or another.
5. The majority of the world, who just want to quietly live their lives in peace and security, would consider money spent on education, infrastructure, medical and scientific research, and other quality of life stuff far better spent than the trillions of dollars spent on war over the past century.
6. It's been said that people get the government they deserve. I think that's nonsense. There is absolutely nothing so bad, so horrible, so despicable that the poor people of Syria and Iraq could possibly have done that would be bad enough that they would deserve what they've got.