Helen cheated on her husband. Agamemnon didn't like being cucked by some fancy-pants Trojan prince, so he decided to take her back and sack the city of Troy while he was about it. Since the spoils of war promised to be rich, he was able to talk many of the rulers of little city states and islands to join him in his effort. He succeeded. Troy was sacked, the ponce who cucked Agamemnon was killed, and a generally good time was had by all the Greeks.