It was the way that WWI ended which, arguably, shaped the rest of the century. The Treaty of Versailles utterly humiliated Germany. Take a nation looking for somebody to get back at, add the abject squalor of the Depression, and toss in the antipathy toward Jews that was common throughout much of Europe for centuries but raised to an artform by the Germans. If Hitler hadn't come along to start WWII, someone else would have.
Without Germany nipping at Stalin's heels, reminding Russia of its frightening encounter with Napoleon, the nation might have devoted more of its GDP to making communism work and less to militarization.
Without WWII, Japan wouldn't have had to withdraw from China and Mao might have been stopped in his tracks. The Republic of China would surely have done to Japan what China did to all of its conquerors from Genghis Khan's Mongols to the Manchus: passively assimilate them, dissipate their might and their gene pool, and rise up again as its Daoist/Confucian/Buddhist self, none the worse for the wear.
The world might be a much different place if the Treaty of Versailles had been toned down a bit.