Final answer:
The primary factor that helped the Allies force Germany to sign the armistice on November 11, 1918 was the massive offensive launched by the Allies, which pushed German forces towards collapse and surrender. Other factors included civil unrest in Germany and the end of the threat posed by Russia on the eastern front.
Step-by-step explanation:
The primary factor that helped the Allies force Germany to sign the armistice on November 11, 1918 was the massive offensive launched by the Allies, which pushed German forces towards collapse and surrender. The arrival of over one million U.S. troops in Europe in 1918 and their participation in major offensives such as the Battle of the Argonne Forest played a significant role in turning the tide of the war in favor of the Allies.
In addition, Germany was facing civil unrest and loss of support from its military high command. The German people in Berlin were in a state of unrest, and the German Emperor Kaiser Wilhelm II abdicated his throne on November 9, 1918. This, coupled with the internal problems Germany was experiencing, further weakened their position and made them more inclined to seek peace.
Finally, the end of the threat posed by Russia on the eastern front was another factor that contributed to Germany's decision to sign the armistice. The Russian Revolution had led to a civil war and the emergence of a new government under Lenin, which led to Russia withdrawing from the war and signing a peace treaty with Germany. This allowed the Allies to focus their efforts on the western front and put additional pressure on Germany.