Germany’s declaration of unrestricted submarine warfare/ sinking of the U.S.A ships
In August, 1914 the United States declared its neutrality in the war then engulfing Europe. President Woodrow Wilson, reflecting the views of much of the nation, announced that his country would be “impartial in thought as well as in action”.
The American entry into World War I came in April 1917, after more than two and a half years of efforts by President Woodrow Wilson to keep the United States out of the war.
The primary events that led to the United States declaration of war against Germany were the Zimmerman Telegram and Germany's announced intention to resume unrestricted submarine warfare.
On February 25, 1917, President Wilson received an intercepted German telegram to the German Embassy in Mexico City that offered to Mexico the "lost territory" of Arizona, New Mexico, etc. (land lost by Mexico to the U.S. in the Mexican American War of 1848) if Mexico would declare war on the U.S. The British had intercepted the telegram, and delivered it to the U.S. in an obvious attempt to secure U.S. entry into the war on the Allied side. On March 17, 1917, Germany sank five U.S. merchant vessels, and President Wilson delivered a war message to Congress on April 2. War was declared on April 6, 1917.