Two reasons the United States finally abandoned its neutrality and chose to enter World War I on the side of the Allies:
- Germany resumed unrestricted submarine warfare.
- The Zimmermann Telegram revealed a German attempt to gain Mexico as an ally against the United States.
Further details:
There had been great outrage against the Germans after the sinking of the British ocean liner, Lusitania, in 1915. In that incident, over 1,000 persons were killed, which included more than 100 Americans. Germany kept the US from entering the war at that time only by pledging to stop submarine attacks. But two years later, the Germans resumed such attacks. This was reason #1 for America to go to war with Germany.
In addition, British intelligence intercepted and decoded a telegram (known as the "Zimmermann Telegram") that showed Germany was trying to secure Mexico as an ally against the United States. The revelation of this plan inflamed feelings in the United States. The telegram was sent in January, 1917. In March, 1917, German Foreign Secretary Arthur Zimmermann admitted the telegram was, in fact, sent by his office.
In April, 1917, the US declared war on Germany in response to the Germans' submarine warfare and the Zimmermann Telegram.
