On April 6, 1917, two days after the U.S. Senate votes 82 to 6 to declare war against Germany, the U.S. House of Representatives endorses the decision by a vote of 373 to 50, and the United States formally enters the First World War.
When World War I erupted in 1914, President Woodrow Wilson pledged neutrality for the United States, a position favored by the vast majority of Americans. 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, calling the attack an unfortunate mistake.
On May 7, the British-owned ocean liner Lusitania was torpedoed without warning just off the coast of Ireland. Of the nearly 2,000 passengers aboard, 1,201 were killed, including 128 Americans. The German government maintained, correctly, 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 a U-boat sank 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 February 1917, Germany, determined to win its war of attrition against the Allies, resumed its policy of unrestricted submarine warfare in war-zone waters. Three days later, the United States broke diplomatic relations with Germany; the same day, the American liner Housatonic was sunk by a German U-boat. On February 22, Congress passed a $250 million arms-appropriations bill intended to ready the United States for war. In late March, Germany sank four more U.S. merchant ships, and on April 2, President Wilson went before Congress to deliver his famous war message. Within four days, both houses of Congress had voted in favor of a declaration of war.
Despite measures taken to improve U.S. military preparedness in the previous year, Wilson was unable to offer the Allies much immediate help in the form of troops; indeed, the army was only able to muster about 100,000 men at the time of American entrance into the war. To remedy this, Wilson immediately adopted a policy of conscription. By the time the war ended on November 11, 1918, more than 2 million American soldiers had served on the battlefields of Western Europe, and some 50,000 of them had lost their lives. Still, the most important effect of the U.S. entrance into the war was economic—by the beginning of April 1917, Britain alone was spending $75 million per week on U.S. arms and supplies, both for itself and for its allies, and had an overdraft of $358 million. The American entry into the war saved Great Britain, and by extension the rest of the Entente, from bankruptcy.
The United States also crucially reinforced the strength of the Allied naval blockade of Germany, in effect from the end of 1914 and aimed at crushing Germany economically. American naval forces reached Britain on April 9, 1917, just three days after the declaration of war. By contrast, General John J. Pershing, the man appointed to command the U.S. Army in Europe, did not arrive until June 14; roughly a week later, the first 14,000 U.S. infantry troops landed in France to begin training for combat. Though the U.S. Army's contributions began slowly, they would eventually mark a major turning point in the war effort and help the Allies to victory.