There are times when you have to hold your nose and partner with someone terrible in order to stop someone perhaps even more terrible. These are the difficult choices that must be made at the highest levels of leadership from time to time.

When Hitler invaded the Soviet Union, FDR expanded the Lend-Lease program and gave billions of dollars worth of military supplies to Stalin. It wasn't like FDR being buddies with Stalin (although I've read that Harry Hopkins was a fan of Stalin), it's just that FDR saw an even bigger threat in Hitler. I think history shows he made the right call.

In the 1980s, Iran was trying to export its Islamic Revolution throughout the Middle East. Had they succeeded, the consequences would've been very bad for the US. The US and several other counties believed that Saddam, as bad as he was, wasn't as bad as the consequences of an Iranian victory. While it's still a bit early to judge the outcome, I think in the long term history will judge that to be a correct history. Of course, to say such a thing, you have to understand history.