German Unrestricted Submarine Warfare: Germany's decision to resume unrestricted submarine warfare, sinking any ship in the war zone (including passenger ships and those of neutral nations), was a significant factor in the U.S. entry into the war. The sinking of the British passenger liner RMS Lusitania in 1915, with the loss of over 100 American lives, sparked outrage and anti-German sentiment in the United States. Germany continued its unrestricted submarine campaign, leading to further losses of American lives and prompting the U.S. to take action.
Zimmerman Telegram: The interception and decoding of the Zimmerman Telegram, a diplomatic message from the German Foreign Secretary, Arthur Zimmermann, added further fuel to the fire. The telegram proposed an alliance between Germany and Mexico, with the promise of returning territories lost by Mexico to the U.S. (such as Texas) in exchange for Mexico entering the war on the side of the Central Powers. The revelation of this secret plot infuriated the American public and solidified support for entering the war against Germany.
These factors, combined with other concerns, such as the protection of American economic interests and the commitment to democratic ideals and freedom of the seas, led the United States to declare war on Germany in April 1917.