April 6, 1917: Two days after the U.S. Senate voted 82 to 6 to declare war against Germany, the U.S. House of Representatives endorses the declaration by a vote of 373 to 50, and America formally enters World War I.

When World War I erupted in 1914, President Woodrow Wilson pledged neutrality for the United States, a position that the vast majority of Americans favored. Britain, however, was one of America’s closest trading partners, and tension soon arose between the United States and Germany over the latter’s attempted quarantine of the British Isles. 

Several U.S. ships traveling to Britain were damaged or sunk by German mines, and in February 1915 Germany announced unrestricted warfare against all ships, neutral or otherwise, that entered the war zone around Britain. One month later, Germany announced that a German cruiser had sunk the William P. Frye, a private American vessel. President Wilson was outraged, but the German government apologized and called the attack an unfortunate mistake.

WATCH: WWI: The Death of Glory on HISTORY Vault

On May 7, the British-owned Lusitania ocean liner was torpedoed without warning just off the coast of Ireland. Of the 1,959 passengers, 1,198 were killed, including 128 Americans. The German government maintained that the Lusitania was carrying munitions, but the U.S. demanded reparations and an end to German attacks on unarmed passenger and merchant ships. In August, Germany pledged to see to the safety of passengers before sinking unarmed vessels, but in November sunk an Italian liner without warning, killing 272 people, including 27 Americans. With these attacks, public opinion in the United States began to turn irrevocably against Germany.

In 1917, Germany, determined to win its war of attrition against the Allies, announced the resumption of unrestricted warfare in war-zone waters. Three days later, the United States broke diplomatic relations with Germany, and just hours after that the American liner Housatonic was sunk by a German U-boat. On February 22, Congress passed a $250 million arms appropriations bill intended to make the United States ready for war. In late March, Germany sunk four more U.S. merchant ships, and on April 2 President Wilson appeared before Congress and called for a declaration of war against Germany. Four days later, his request was granted.

On June 26, the first 14,000 U.S. infantry troops landed in France to begin training for combat. After four years of bloody stalemate along the western front, the entrance of America’s well-supplied forces into the conflict marked a major turning point in the war and helped the Allies to victory. When the war finally ended, on November 11, 1918, more than two million American soldiers had served on the battlefields of Western Europe, and some 50,000 of them had lost their lives.

READ MORE: 8 Events Leading to the Outbreak of World War I