Associated Press
In this undated photo, US infantrymen advance on the St. Mihiel front.
When the US declared war on Germany on April 6, 1917, the Allies had already endured over three years of carnage.
The famed Harlem Hellfighters were the first to join the fight on the frontlines under French command.
As the Americans began to send more troops across the Atlantic, Germany was anxious to score a victory before the majority of their forces arrived.
In the midst of the Bolshevik Revolution, Russia could not sustain its war effort. The dissolution of its forces freed German soldiers to the east, and the Central Powers' ranks along the western front swelled.
The plan for Kaiserschlacht, or "Kaiser's battle," was to conquer Paris, and end the war.
That plan backfired, and with the additional US troops, the Allies were able to counter a string of German offensives, starting with the first US-led assault in a small village near the Somme called Cantigny.