How did the final year of the Great War and the subsequent Peace Conference pave the way for a very violent future for both Europe and the United States?

By the summer of the last year of the war, 1918, the United States was sending 10,000 new troops a day to France. The Allies therefore had many more troops and were also able to greatly outspend the Central Powers. When the Allies launched what was called the hundred days offensive in August, 1918, the German High Command knew it could not win the war, and soon thereafter the Central Powers began to make peace overtures.
The Germans surrendered but their army was still robust, even with many troops suffering from influenza, and the Allied troops did not invade Germany. Because the German army was not annihilated nor the country invaded, this led later to the "backstab" theory that a still strong Germany had been sold out by the Jews and the Bolsheviks. In addition, the Versailles Treaty, which was foisted on the Germans against their will, forced them to assume complete responsibility for the war and bear all the costs. The treaty was so humiliating that even at the time of its signing, people were predicting it would lead to another war.
The "backstab" theory gave Hitler a route to power, as did the deeply resented Versailles Treaty. Once his militaristic dictatorship was in place in Germany a second, very violent war was guaranteed, especially given Hitler's desire for world conquest.