On May 7, 1915, the British passenger liner RMS Lusitania was sunk by a German U-boat, killing 1,198 people, including 128 Americans. The sinking outraged Americans and increased support for joining the war against Germany.
2. 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 that Mexico and Germany should form an alliance against the United States. The telegram further stated that Germany would support Mexico in reconquering its lost territories in the American Southwest. The Zimmermann Telegram convinced many Americans that Germany was a threat to the United States and that the country should enter the war.
3. Resumption of unrestricted submarine warfare
In February 1917, Germany announced that it would resume unrestricted submarine warfare in the Atlantic Ocean. This meant that German U-boats would now attack any ship, including passenger ships, without warning. The resumption of unrestricted submarine warfare increased the risk to American ships and led to the deaths of several American citizens.
4. American public opinion
Public opinion in the United States was strongly in favor of joining the war against Germany by early 1917. The sinking of the Lusitania, the Zimmermann Telegram, and the resumption of unrestricted submarine warfare had all contributed to this shift in public opinion. In addition, many Americans were concerned about the threat of a German victory and the possible consequences for the United States.