Germany used submarines to attack ships without warning. In 1915, the sinking of the British ship Lusitania, which killed 128 Americans, angered the U.S. government.

German submarines, known as U-boats, began attacking ships in the Atlantic Ocean, including those carrying American citizens and goods. A significant event was the sinking of the British passenger ship, the Lusitania, in 1915, which resulted in the deaths of 128 Americans. This incident angered the American public and increased calls for action against Germany.

Before and during World War I, the United States maintained significant economic relationships with European nations, particularly Britain and France. American banks had invested heavily in these Allied nations, providing substantial loans that totaled billions of dollars. Additionally, American businesses conducted extensive trade with these countries, exporting crucial supplies, raw materials, and manufactured goods. This economic interdependence meant that an Allied defeat could devastate the American economy, as it would jeopardize loan repayments and disrupt established trade relationships.

The economic ties between the United States and the Allied powers grew stronger as the war progressed. American manufacturers increased their production of war materials, including weapons, ammunition, and other military supplies sold to Britain and France. American agricultural exports to these nations also rose dramatically, as European farmers had left their fields to fight in the war. These economic factors created a situation where American prosperity became increasingly dependent on an Allied victory, leading many American business leaders and bankers to advocate for U.S. support of the Allied cause. This financial relationship played a crucial role in shaping American public opinion and ultimately influenced the nation's decision to enter the war in 1917.

In 1917, British intelligence intercepted a secret message from Germany to Mexico. In this telegram, Germany proposed a military alliance with Mexico if the U.S. entered the war. If Mexico agreed, Germany promised to help Mexico regain territories lost to the U.S., such as Texas, New Mexico, and Arizona. When this information became public, it shocked many Americans and pushed them further toward supporting the war.

When the United States entered World War I in 1917, the nation faced the monumental task of transforming its small peacetime military into a massive fighting force. Affectionately nicknamed "doughboys," American troops underwent rapid training before being shipped overseas to join the Allied Powers. The American Expeditionary Forces (AEF), led by General John J. Pershing, grew from approximately 150,000 soldiers in 1917 to over 4.7 million by the war's end in 1918, marking one of the most significant military mobilizations in U.S. history.

The American troops played a crucial role in turning the tide of the war, particularly in major battles such as Cantigny, Belleau Wood, and the Meuse-Argonne Offensive. Despite their late entry into the conflict, U.S. forces helped break the stalemate on the Western Front, contributing fresh resources and manpower when Allied forces were exhausted. American soldiers faced numerous challenges, including trench warfare, chemical weapons, and the Spanish Flu pandemic, yet their participation proved decisive in securing Allied victory. By the war's end, approximately 116,516 American military personnel had lost their lives, with another 204,002 wounded, marking a significant sacrifice in the nation's first major international military intervention of the 20th century.

During World War I (1914-1918), the United States played a crucial role as a supplier of resources and materials, even before officially entering the war in 1917. American industry underwent a massive transformation to meet the demands of both domestic and Allied forces. The nation's factories shifted from civilian production to manufacturing military supplies, including weapons, ammunition, vehicles, and other essential war materials. This industrial mobilization was unprecedented in American history, with factories working around the clock to produce everything from rifles and artillery shells to uniforms and medical supplies.

The United States' vast natural resources proved invaluable to the war effort. American farms dramatically increased production to feed U.S. troops, Allied forces, and European civilian populations. The nation's abundant coal and oil reserves powered the industrial machine, while its steel mills produced the metal needed for weapons and vehicles. The War Industries Board, established in 1917, coordinated this massive production effort, ensuring that raw materials and manufactured goods reached their intended destinations efficiently. This period marked the beginning of America's emergence as a global industrial and military superpower, demonstrating its capacity to mobilize its vast resources and industrial might for war purposes.

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The United States' entry into World War I in 1917 brought a significant advantage to the Allied Powers - fresh, well-rested troops who had not experienced the physical and mental exhaustion of years of trench warfare. While European soldiers had been fighting since 1914, dealing with the harsh conditions of prolonged combat, disease, and psychological trauma, American forces arrived with high morale and full strength. This injection of fresh energy and resources came at a crucial time when many European armies were experiencing severe war fatigue and diminishing resources.

The impact of these fresh American troops became evident in several key battles that helped turn the tide of the war. At the Battle of Cantigny in May 1918, American forces demonstrated their capabilities by successfully capturing and holding the village against German counterattacks, marking the first American offensive of the war. Later, during the Meuse-Argonne Offensive (September-November 1918), the largest American military operation in history, these well-rested troops played a decisive role. The American forces' energy, determination, superior numbers, and fresh supplies helped break through German defenses in what would become one of the final pushes that led to the war's end. This offensive demonstrated how American troops' fresh perspective and strength could overcome even well-established defensive positions, contributing significantly to the Allied victory.

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