We'll be honest. We're not exactly sure who or what is supposed to function as the main antagonist to our boy Troilus. Sure, we could say that Cressida cheats on him and breaks his heart. But Cressida's behavior is a bit more complicated than that, don't you think?
So, maybe we should point out that Diomedes is the one who steals Cressida away and rubs it in Troilus' face on the battlefield, because that is most definitely antagonistic. Still, that doesn't seem quite right, because we get the feeling that if Cressida didn't cheat with Diomedes, then it would have been someone else.
Should we just go ahead and blame the whole Trojan War or politics in general? After all, Troilus and Cressida are separated from each other because of some political maneuvering they have no control over. Or, maybe we should just say that Fate in general—or even history or literature—is the antagonist. After all, before Shakespeare even picked up his pen to write this play, Troilus was doomed to be betrayed by a girl named Cressida. What do you think?