The primary factor that led the United States to join the fight against Germany in World War I was the resumption of unrestricted submarine warfare by the German Empire in February 1917. This policy involved the sinking of merchant ships without warning, regardless of their nationality, in an attempt to cut off vital supplies to Britain and its allies.
Other factors that contributed to the US decision to enter the war included:
* The sinking of the British passenger liner RMS Lusitania in May 1915, which resulted in the deaths of over 1,000 civilians, including 128 Americans.
* The interception and publication of the Zimmermann Telegram, a secret diplomatic message from the German Foreign Secretary Arthur Zimmermann to the German ambassador in Mexico, proposing an alliance between Germany and Mexico against the United States.
* The increasing concern about the spread of communism in Russia following the Bolshevik Revolution in November 1917.
The United States declared war on Germany on April 6, 1917, and played a significant role in the Allied victory in the war.