With a POD no earlier than the beginning of first world war, and preferably no earlier than the 1918 Armistice, prevent the post-war decline of liberal democracy and the rise of Nazism, Fascism and Communism.
I've found this to be harder than it sounds. A less strenuous treaty of Versailles may have resulted in no Nazis but that still leaves Italian fascism, Soviet communism, and Japanese imperialism. Italian Fascism and Soviet Communism were both solidified in the immediate aftermath of the war, while Japan's imperial expansion had already begun prior to the beginning of the first world war.
I've found this to be harder than it sounds. A less strenuous treaty of Versailles may have resulted in no Nazis but that still leaves Italian fascism, Soviet communism, and Japanese imperialism. Italian Fascism and Soviet Communism were both solidified in the immediate aftermath of the war, while Japan's imperial expansion had already begun prior to the beginning of the first world war.
Perhaps a less isolationist United States and a more muscular League of Nations could have contained the rise of totalitarianism, but even then it seems as though the head-winds of history were dead-set against liberalism in the post-WW1 era. The fact is, authoritarian movements sprung up somewhat independently of each other.