What was the Oath of Tyndareus?

Answer 1

It was the oath that Helen's father, Tyndareus, King of Sparta, made all of her suitors take before he would announce whom he had chosen for her husband. Since so many kings had come to court Helen (who chose Menelaus herself, although he was not present), Tyndareus was afraid to accept any gifts or name any favorite for fear of giving any of those present a reason to quarrel with him. Odysseus, who knew he never had a chance, offered to help Tyndareus of the Spartan king would support his bid to marry Penelope, daughter of King Icarius. Tyndareus agreed and the oath stated that all present would ever come to the defense of the chosen husband if anyone brought quarrel to him. This would prevent any one kingomd from trying to attack Sparta because all of the other kingdoms would be bound to come to their aid.

It should be noted that no Trojans were present at the courting of Helen, and that the Oath is one of the driving factors of the Trojan War.