Attacking Adolf Hitler's Germany benefited moral monster Joseph Stalin's Soviet Union. That inescapable fact of World War II doesn't mean it wasn't worth fighting Nazis. Under the circumstances, allying with Stalin to beat Hitler was the right call. But the consequences of that fraught alliance were themselves horrific. To ignore its downsides would be to misunderstand the real choices America faced.
Today in Syria, the United States faces another set of harrowing tradeoffs. Syria's leader, Bashar al-Assad, is a murderous dictator. The various rebel groups trying to wrest the country from his control include ISIS, a radical Sunni militia. To strike Assad, as Obama threatened to do last summer, would help ISIS. To strike ISIS, as the Obama administration is threatening to do right now, would help Assad.