The relative peace during the 99 years after the Treaty of Vienna was shattered in 1914 by World War I. It was set off by a Serbian nationalist assassinating Archduke Franz Ferdinand, the heir to the Austro-Hungarian Throne. Germany, which was rising economically and militarily to challenge Britain’s dominance, backed Austria-Hungary. So did the Turkish Ottoman Empire. On the other side, Serbia was backed by Russia, France and Great Britain.
The war erupted for many reasons besides the assassination. There was growing colonial and nationalist competition among the various powers, and interlocking alliances led to a chain reaction of mobilizations between the two sides. The Germans over-optimistically calculated they could first defeat France and then Russia in quick succession, securing a stronger position on the European continent.
Instead of what everyone thought would be a quick and exciting adventure, there were years of trench warfare and traditional frontal assaults against modern weapons such as machine guns, poison gas and long-range, rapid-fire artillery. There were 10 million deaths, sometimes as many as 20,000 in a single day.
After three years of seesaw results and stalemate, Germany was weakening and becoming desperate. Its submarines began sinking U.S. ships that were supplying Britain. They hoped to starve the British into surrender before the Americans came in. Instead, the United States came in on the Allied side (Britain/France/Russia), tilting the balance with more troops and supplies. After a last-gasp offensive stalled, an exhausted Germany realized they could not win and decided to surrender. So the U.S. supplied the allies and tilted the balance.
In the aftermath of the war, the Russian, Austro-Hungarian, German and Turkish governments were overthrown and all the European economies were fatally weakened.
With his idealistic Fourteen Points (national self-determination, no secret treaties, etc.), U.S. President Woodrow Wilson attempted to establish a new, peaceful postwar international order at the 1919 Treaty of Versailles peace talks, most importantly by trying to negotiate a fair surrender and by establishing the League of Nations (the second modern attempt to avoid war with a collective security organization).
However, the Fourteen Points was sabotaged when France and Britain imposed a harsh peace that was more typical of the era, in order to destroy German military and economic power. Germany lost its overseas colonies and had to disarm, take blame for the war, and pay reparations. Meanwhile, the League of Nations was weakened when isolationists in the U.S. Senate didn’t ratify the League Treaty (they felt it would violate U.S. sovereignty) and the U.S. didn’t join.