The United States' entry into World War I was precipitated by a series of aggressive actions from Germany, particularly the declaration of unrestricted submarine warfare that led to the sinking of several American merchant vessels. In early 1917, U.S. ties with Germany deteriorated further following the revelation of the Zimmerman Telegram, in which Germany sought a military alliance with Mexico against the U.S. These provocations spurred public and political sentiment towards war, shifting the longstanding American policy of neutrality. President Woodrow Wilson, who initially aimed to keep the United States out of the European conflict, found growing support for his administration's interventionist stance.

On April 2, 1917, President Wilson delivered a speech to Congress requesting a declaration of war against Germany, stating that the world must be made safe for democracy. The call resonated with many, and after extensive debate, Congress voted overwhelmingly in favor of war on April 6, 1917. This was a pivotal moment in American history, marking the country's transition to an active role in global affairs and the processes of international politics.

As the U.S. mobilized for war, it also faced the challenge of quickly building an army and preparing for combat overseas, which would ultimately lead to a significant American presence in Europe. With U.S. forces engaged in the conflict by 1918, the entry of American troops was a decisive factor that contributed to the eventual defeat of the Central Powers and the end of the war.