The United States entered the First World War after Congress declared war on the German Empire.
American President Woodrow Wilson had made his case for war before a special joint session of Congress four days earlier. During his address he claimed that, ‘we have no selfish ends to serve’ since the war would ‘make the world safe for democracy’. American involvement was, therefore, a moral obligation.
Wilson’s decision to request a declaration of war came in the wake of two significant developments against the United States. Firstly, Germany’s decision to resume unrestricted submarine warfare meant that all ships, including those from neutral countries, which traded with Germany’s enemies were now a permissible target for sinking. Secondly, the British passed on the intercepted contents of a secret telegram from the German Foreign Secretary, Arthur Zimmermann, to the Mexican government. The telegram sought a German alliance with Mexico in the event of the United States declaring war. Significantly, it promised that Arizona, Texas and New Mexico would return to Mexican control once victory had been achieved.
Wilson had initially been reluctant to declare war due to his belief that public opinion in the United States was against it. However, with civilian merchant and passenger ships once again under threat, and with the Zimmermann telegram’s indication of German intentions to attack the United States itself, he felt that the American public were ready to change their minds.
Following Wilson’s address to the joint session, the Senate and the House met separately to debate and vote on the resolution. The Senate passed it by 82 votes to 6, while the House of Representatives did so by 373 to 50.