In many ways, World War I was what truly ended the nineteenth century. It undermined the faith in progress that had grown, despite all of its setbacks, throughout the nineteenth century among many, perhaps most, Europeans. The major political movements of the nineteenth century seemed to have succeeded: everywhere in Europe nations replaced empires (nationalism). Europe controlled more of the world in 1920 than it ever had or ever would again (imperialism). In the aftermath of the war, almost every government in Europe, even Germany, was a republican democracy based on the rule of law (liberalism). Even socialists had cause to celebrate: there was a nominally Marxist state in Russia and socialist parties were powerful and militant all across Europe.
Thumbnail: Benito Mussolini with Hitler on 25 October 1936, when the axis between Italy and Germany was declared. (Public Domain; via Wikipedia)