Why were the generals of Troy obligated to defend Helen?

When it was time for Helen to find a husband, there was much competition to see who would recieve her hand in marriage. King Tyndareus, her father, feared that war might break out over the fight for Helen, so he found a way to ensure this would not happen. Odysseus, in exchange for help in winning King Tyndareus's niece's hand in marriage, told the king of a way to stop war from breaking out. Thus, the Oath of Tyndareus was made and taken by all of Helen's suitors, who promised they would protect whoever married Helen, and thus protect Helen.