don't sink my ship, i sting

When Helen was of age to be married, all the Lords of Greece came to seek her hand. This was an issue for her stepfather the King of Sparta who could only pick one husband and then would have to deal with the far greater number of angry rejected suitors.

One suitor was Odysseus, then a younger man. He quickly ascertained that he had no chance of marrying Helen. Ithaca was too poor for him to even bring any proper gifts, let alone a dowry. But he did catch the eye of the King's niece, one Penelope.

Now Penelope was still a bit out of Odysseus' league, but not so much as Helen was. So he makes a deal with the King of Sparta. Support my bid to marry Penelope, and I'll give you a solution to this suitors problem. The King agrees, so Odysseus suggests all the suitors swear an oath to defend Helen's marriage from anyone who might strike against it. They all, including Odysseus, so swear, and Helen is married to Menelaus. The King proves as good as his word, and Penelope is wed to Odysseus.

Of course this means that Odysseus has now bound himself to fight for Helen and Menelaus. After an attempt to feign madness he is forced to fulfill his duty.