The Zimmermann Telegram was a 1917 diplomatic proposal from the German Empire for Mexico to join an alliance with Germany in the event of the United States entering World War I against Germany. The proposal was intercepted and decoded by British intelligence. The revelation of the contents outraged the American public and swayed public opinion. President Woodrow Wilson moved to arm American merchant ships to defend themselves against German submarines, which had started to attack them. The news helped generate support for the United States declaration of war on Germany in April of that year.