Sixtus spent most of his reign involved in local Italian squabbles. He made other forays into international politics - a half-hearted crusade against the Turks, small gestures towards unification with the Greek Orthodox church - but these didn't really go anywhere.

We can note, however, that he was the pope who officially sanctioned the Inquisition*, under political pressure from some powerful Spaniards. He didn't expect it to go as far as it did, however, and expressed regret at their exuberance.

*Who expected that? No one, that's who.