As big-shot Shakespeare scholar Marjorie Garber says, Troilus blows off the Trojan War while he tries to get Cressida into bed. Meanwhile, the Trojan military leaders want to get the great warrior Achilles out of bed and back onto the battlefield. And that, Shmoopers, is what we call an Act I set up. (Okay, okay, even though what we've got here is really five acts.)
Troilus finally sleeps with Cressida, but the next morning, she's dragged off to the Greek camp after being traded for a Trojan warrior. Meanwhile, the Trojan military leaders try to make Achilles jealous of another warrior so he'll put his armor back on and join the fight. Things are looking really bad, guys.
Cressida betrays Troilus about 2 seconds after arriving at the Greek camp, where she agrees to sleep with Diomedes. Troilus is so enraged that he goes into battle looking to kill the guy who stole his girl. When Hector kills Achilles's lover on the battlefield, Achilles roars into action and slaughters the guy responsible for his lover's death. Moral of the story? Love makes people do really stupid things. Remember that.