Final answer:
The United States joined the war for the Allies due to German aggression, unrestricted submarine warfare, and the Zimmermann Telegram, which threatened US sovereignty and galvanized public opinion towards war.
Step-by-step explanation:
The factors that helped prompt the United States to join the war for the Allies were German aggression, unrestricted submarine warfare, and the Zimmermann Telegram. The sinking of US ships by German U-boats angered the American public and challenged the US's position of neutrality. The final straw was the Zimmermann Telegram, which was a secret communication from the German foreign minister to the ambassador in Mexico, proposing a military alliance between Germany and Mexico against the United States, with promises to help Mexico regain territories lost during the Mexican-American War. This egregious breach of neutrality and direct threat to US sovereignty created a surge in public opinion in favor of war, eventually leading to the decision to join the Allies in World War I.