Final answer:
The Oath of Tyndareus bound Greek princes to defend Helen of Troy's husband, leading them to support him during the Trojan War.
Step-by-step explanation:
In Greek mythology, the Oath of Tyndareus was a solemn agreement that bound the Greek princes to defend and protect the chosen husband of Helen of Troy. This oath compelled all the suitors of Helen to pledge that they would support the eventual husband in any quarrel related to her. As a result, when Helen was taken to Troy by Paris, sparking the Trojan War, the Greek princes, bound by the Oath of Tyndareus, were obligated to join forces to retrieve her, thereby leading to a massive Greek expedition against Troy.