The Sinking of Lusitania: In May 1915, a German submarine sank the British passenger ship Lusitania, killing over 1,200 people, including 128 Americans. The incident outraged public opinion in the United States and increased support for intervention in the war.
The Zimmermann Telegram: In early 1917, the British intercepted and decoded a telegram from German Foreign Minister Arthur Zimmermann to the German ambassador in Mexico. The telegram proposed an alliance between Germany and Mexico, with Germany offering to support Mexico in recovering territories lost to the United States in the Mexican-American War. The disclosure of the telegram further inflamed public opinion in the United States and led many to believe that Germany was a threat to American security.
The Sussex Pledge: In May 1916, Germany pledged not to sink merchant ships without warning, following several incidents involving American vessels. However, Germany resumed unrestricted submarine warfare in early 1917, leading Wilson to break diplomatic relations with Germany.
The Unrestricted Submarine Warfare: In February 1917, Germany announced a resumption of unrestricted submarine warfare in the Atlantic, ignoring the Sussex Pledge. This action threatened American shipping and increased the risk of direct conflict between the United States and Germany.
The Russian Revolution: The February Revolution in Russia in 1917 and the subsequent overthrow of the Tsarist regime had a significant impact on Wilson's thinking about the war. He believed that the United States had a responsibility to support democratic movements abroad and help spread democracy throughout the world.
In summary, although Woodrow Wilson wanted to maintain a policy of neutrality, a combination of factors, including the sinking of the Lusitania, the Zimmermann Telegram, Germany's resumption of unrestricted submarine warfare, and the Russian Revolution, eventually led him to change his opinion and enter World War I on the side of the Allies.