1. Unrestricted submarine warfare: The immediate trigger for America's entry into the war was the German government's decision to resume unrestricted submarine warfare in February 1917. This policy meant that German submarines would sink any ship, including passenger ships, without warning. This policy led to the sinking of several American ships and the deaths of American citizens.
2. The Zimmerman Telegram: In January 1917, the British intercepted a telegram from the German foreign secretary, Arthur Zimmermann, to the German ambassador in Mexico. The telegram proposed an alliance between Germany and Mexico in the event of war between the United States and Germany. The telegram also promised Mexico the return of territories lost to the United States in the Mexican-American War. The telegram outraged American public opinion and helped to turn the tide of public sentiment in favor of war.
3. Economic ties: The United States had close economic ties with Britain and France, and the Allied Powers owed the United States billions of dollars in loans. If the Allies lost the war, the United States would have lost a significant amount of money.
4. Ideological differences: The United States and Germany had very different political and ideological systems. The United States was a democracy, while Germany was a monarchy. The United States also believed in the spread of democracy around the world, while Germany believed in the dominance of the German race.
5. American public opinion: Public opinion in the United States was divided on the issue of whether or not to enter the war. However, after the sinking of the Lusitania and the Zimmerman Telegram, public opinion shifted in favor of war.
In the end, the United States entered World War I in order to protect its own interests and to help to ensure the victory of the Allies.