The World War I had started in 1918. It was mostly fought between the German Empire and the allied forces. The allied forces comprised of France and Britain on the western front of the war.
Later when Germany attacked American ships in 1917, USA decided to enter the war on the side of the allied powers. So in 1918, the American soldiers were fighting alongside British and French troops on the western front.
In September, 1918, the allied forces decided to launch a major assault against the German front-lines. By this time, the allied forces had scored many important victories while the German army was drawing near to exhaustion. Also, a large portion of the American army had not been engaged in the war yet, and the allied powers intended to use them in order to bring the war to a quick end.
So the Meuse-Argonne Offensive was launched, later known as the Hundred Days Offensive. During this offensive, the allied forces attacked the German lines all along the western front. The offensive was the largest operation of the American Expeditionary Forces (AEF) during World War I.
The Meuse-Argonne Offensive began on September 26, 1918. And it concluded on November 11, 1918, with the defeat of the German Empire. The offensive took place all along the western front of the war
The American forces were commanded by General John J. Pershing. He commanded nearly 1.2 million American soldiers who took part in this offensive. More than 50,000 French troops also fought on the allied side. Notable French commanders included Henri Gouraud and Henri Berthelot.
The German forces were commanded by Georg von der Marwitz and Max von Gallwitz. These commanders led a force of around 450,000 troops.
The purpose of the Meuse-Argonne Offensive was to overwhelm the German forces and to conclude the war once and for all. So the American forces attacked the German lines in the south on September 26 in a major offensive. The next day, British and Belgian forces launched another major offensive on the Belgian front. The French and British forces launched a major assault on the German positions in northern France.
The huge number of American army, at around 1.2 million, was a decisive factor in the outcome of the Meuse-Argonne Offensive. Although most of the American troops were inexperienced in the kind of warfare that took place during the World War I, they learned quickly and were able to secure many crucial victories for the allied forces.
The Germans stiffly resisted the offensive but they were soon overwhelmed the numerical superiority of the allied forces. The American troops alone outnumbered the Germans by 2 to 1. So under the large-scale assault, the German defenses started collapsing. By November, 1918, Germans had lost most of their defensive positions along the western front and were retreating. So the German Empire agreed to an armistice, bringing the war to an end.