The United States originally pursued a policy of non-intervention, avoiding conflict while trying to broker a peace. When a German U-boat sank the British liner Lusitania in 1915, with 128 Americans aboard, U.S. President Woodrow Wilson vowed, "America is too proud to fight" and demanded an end to attacks on passenger ships. Germany complied. Wilson unsuccessfully tried to mediate a settlement. He repeatedly warned the U.S. would not tolerate unrestricted submarine warfare, in violation of international law and U.S. ideas of human rights. Wilson was under pressure from former president Theodore Roosevelt, who denounced German acts as "piracy". Wilson's desire to have a seat at negotiations at war's end to advance the League of Nations also played a role. Wilson's Secretary of State, William Jennings Bryan, resigned in protest at what he felt was the President's decidedly warmongering diplomacy. Other factors contributing to the U.S. entry into the war include the suspected German sabotage of Black Tom in Jersey City, New Jersey, and the Kingsland Explosion in what is now Lyndhurst, New Jersey.

Making the case

In January 1917, after the Navy pressured the Kaiser, Germany resumed unrestricted submarine warfare. Britain's secret Royal Navy cryptanalytic group, Room 40, had broken the German diplomatic code. They intercepted a proposal from Berlin (the Zimmermann Telegram) to Mexico to join the war as Germany's ally against the United States, should the U.S. join. The proposal suggested that if the U.S. were to enter the war then Mexico should declare war against the United States and enlist Japan as an ally. This would prevent the United States from joining the Allies and deploying troops to Europe, and would give Germany more time for their unrestricted submarine warfare program to strangle Britain's vital war supplies. In return, the Germans would promise Mexico support in reclaiming the territories of Texas, New Mexico, and Arizona that Mexico lost during the Mexican-American War 70 years earlier.

U.S. declaration of war on Germany

After the British revealed the telegram to the United States, President Wilson, who had won reelection on his keeping the country out of the war, released the captured telegram as a way of building support for U.S. entry into the war. He had previously claimed neutrality, while calling for the arming of U.S. merchant ships delivering munitions to combatant Britain and quietly supporting the British blockading of German ports and mining of international waters, preventing the shipment of food from America and elsewhere to combatant Germany. After submarines sank seven U.S. merchant ships and the publication of the Zimmerman telegram, Wilson called for war on Germany, which the U.S. Congress declared on 6 April 1917.

Crucial to U.S. participation was the sweeping domestic propaganda campaign executed by the Committee on Public Information, overseen by George Creel. The campaign consisted of tens of thousands of government-selected community leaders giving brief carefully scripted pro-war speeches at thousands of public gatherings. Along with other branches of government and private vigilante groups like the American Protective League, it also included the general repression and harassment of people either opposed to American entry into the war or of German heritage. Other forms of propaganda included newsreels, photos, large-print posters, magazine and newspaper articles, and so forth. Additionally, during World War I, Woodrow Wilson placed a great importance on children, especially the Boy Scouts of America, asking them to encourage war support and educate the public about the importance of the war. They helped distribute these war pamphlets, helped sell war bonds, and helped to drive nationalism and support for the war.

First active U.S. participation in World War I
The United States was never formally a member of the Allies but became a self-styled "Associated Power". The United States had a small army, but, after the passage of the Selective Service Act, it drafted 2.8 million men and by summer 1918 was sending 10,000 fresh soldiers to France every day. In 1917, the U.S. Congress gave U.S. citizenship to Puerto Ricans when they were drafted to participate in World War I, as part of the Jones Act. Germany had miscalculated, believing it would be many more months before they would arrive and that the arrival could be stopped by U-boats. The United States Navy sent a battleship group to Scapa Flow to join with the British Grand Fleet, destroyers to Queenstown, Ireland and submarines to help guard convoys. Several regiments of U.S. Marines were also dispatched to France. The British and French wanted U.S. units used to reinforce their troops already on the battle lines and not waste scarce shipping on bringing over supplies. The U.S. rejected the first proposition and accepted the second. General John J. Pershing, American Expeditionary Force (AEF) commander, refused to break up U.S. units to be used as reinforcements for British Empire and French units. As an exception, he did allow African-American combat regiments to be used in French divisions. The Harlem Hellfighters fought as part of the French 16th Division, earning a unit Croix de Guerre for their actions at Chateau-Thierry, Belleau Wood and Sechault. AEF doctrine called for the use of frontal assaults, which had long since been discarded by British Empire and French commanders because of the large loss of life.