Germany's declaration of war on the U.S. was the most damaging event to American neutrality in World War I because it directly involved the United States in the conflict. Prior to this declaration, the U.S. maintained a stance of neutrality, but once Germany declared war, it prompted the U.S. to mobilize troops and resources to support the Allies. This shift not only marked a significant turning point in the war but also highlighted the tensions and provocations that had been building between the U.S. and Germany, including unrestricted submarine warfare and the interception of the Zimmerman Telegram, which sought to entice Mexico into the war against the U.S. Ultimately, the U.S. entry into the war played a crucial role in the eventual Allied victory.