Final answer:
The United States entered WWI due to the unveiling of the Zimmermann Telegram, which suggested a German-Mexican alliance against the U.S., and due to Germany's unrestricted submarine warfare sinking American vessels. This led to the U.S. declaration of war on April 6, 1917.
Step-by-step explanation:
The United States entered World War I primarily due to two significant reasons: the Zimmermann Telegram and submarine warfare. The Zimmermann Telegram was a secret message sent by German Foreign Minister Arthur Zimmermann to the German ambassador in Mexico, proposing a military alliance against the United States should it join the war on the side of the Allies. It also promised Mexico the territories of Texas, New Mexico, and Arizona upon victory. This revelation exacerbated the already tense relations between the United States and Germany.
Another key factor was Germany's resumption of unrestricted submarine warfare, with German U-boats sinking several American ships. The combination of this aggressive act and the Zimmermann Telegram's contents provoked a wave of public anger and ultimately convinced the U.S. government that it needed to enter the war to preserve American honor and ensure national security.
Despite some resistance, including the largest 'no' vote against a war resolution in American history, Congress declared war on Germany on April 6, 1917.