Answer:
The United States joining the Allies.
Explanation:
From 1814 to 1917, neither side, Allies and Central Powers, were able to outmatch its opposing forces, which had resulted in a stalemate. And even though the U.S. officially entered the war on the Allied side in 1917, U.S. troops would be able to arrive in the battlefield until 1918. On the other hand, the Germans had forced Russia out of the war and a great number of troops that had been fighting the Russians were now freed and would arrive in the Western Front to fight against the British and French in the spring of 1918. As German military leaders clearly knew that these reinforcements would only temporarily give the German army an edge against the Allies, as the U.S. expeditionary forces were expected to arrive soon, an all-out offensive was planned and launched in the spring of 1918. At first, the troop reinforcements and use of new tactics resulted in great progress, but after a relatively short time the Allies managed to slow down and eventually stop the German offensive. Shortly after that, the U.S. troops arrived in France along with large amounts of supplies, whereas the German military economy was on the verge of collapsing. At this moment, the German High Command realized that the war was no longer winnable and could only manage to resist until the spring of 1919, albeit at a great cost of lives, as the German economy could not match the huge economic power of the United States, so they decided to request an armistice in November 1918, before more lives were lost uselessly.
