A friend sent this along:
We support the Iraqi government in the fight against ISIS.
We don’t like ISIS, but ISIS is supported by Saudi Arabia who we do like.
We don’t like Assad in Syria. We support the fight against him, but ISIS is also fighting against him.
We don’t like Iran, but Iran supports the Iraqi government in its fight against ISIS.
So some of our friends support our enemies, some enemies are now our friends, and some of our enemies are fighting against our other enemies, who we want to lose, but we don’t want our enemies who are fighting our enemies to win.
If the people we want to defeat are defeated, they could be replaced by people we like even less.