When World War I began in Europe in 1914, the United States initially pursued a policy of neutrality. President Woodrow Wilson prioritized diplomatic efforts and sought to keep the country out of the conflict, reflecting widespread public reluctance to become entangled in foreign wars. However, growing economic connections with Allied powers and increasing maritime confrontations gradually shifted U.S. stance toward involvement.

A significant turning point was Germany’s adoption of unrestricted submarine warfare, which targeted commercial and passenger ships crossing the Atlantic Ocean. This policy led to the sinking of the British ocean liner RMS Lusitania in 1915, resulting in the deaths of 128 American citizens. The incident heightened tensions between the two nations. Relations further deteriorated with the interception of the Zimmermann Telegram, in which Germany proposed a military alliance with Mexico against the United States.

In response to these developments, the U.S. Congress formally declared war on Germany in April 1917, marking America’s entry into World War I. The United States mobilized swiftly, assembling over two million troops known as the American Expeditionary Forces (AEF), commanded by General John J. Pershing. These fresh forces arrived on the Western Front, bolstering exhausted Allied troops and playing a pivotal role in key campaigns such as the Meuse-Argonne offensive. The infusion of American manpower contributed significantly to shifting momentum in favor of the Allies.

Domestically, the war effort brought substantial changes to American society. Industrial production expanded rapidly to supply weapons, vehicles, and other military materiel. Women joined the workforce in large numbers to fill positions vacated by men serving overseas. The government undertook extensive propaganda initiatives to maintain public support, including war bond campaigns and the activities of agencies like the Committee on Public Information. At the same time, civil liberties were curtailed through legislation such as the Espionage Act of 1917, reflecting concerns about dissent and national security.

By November 1918, an armistice brought an end to hostilities. President Wilson played a prominent role in the subsequent peace negotiations, advancing his vision for a League of Nations aimed at preventing future conflicts. Although the United States ultimately chose not to join the League, its participation in World War I marked a decisive turning point. The conflict established the country as a significant global power and reshaped its engagement in international affairs.