I listened to a lecture about the modernist movement in literature today. It said that, in the 1930s, academia and literature became very politicized, as everyone had to take sides in the liberal/fascist/communist debate.
One of the key distinctions between modernism and post-modernism is that modernism, as exemplified in Modernist architecture, is ideological, and claims to have the final viewpoint based on scientific principles, whereas postmodernism refuses to believe that we should follow any one ideology exclusively. Postmodernism thus tends to focus on surface issues, notably in literature and art, where it becomes art about art.
But when looking at art, you really can't make any useful distinctions between works classified as post-modern, like a Jackson Pollock painting or a William Burroughs book, and works classified as modernist, like a Picasso, or TS Eliot, or James Joyce's later work. Also, Nietzsche and Freud are the most-commonly-cited ancestors of post-modernism, and they wrote in the 19th century. Frederick Taylor is a commonly-cited ancestor of modernism, and he did his work in the early 20th century. And the Dadaists did everything that post-modern artists did, fifty years earlier.
I think we can revise the timeline: Post-modernism actually came before modernism. Then modernism intervened, largely because of the polarizing politicization of art in the time leading up to and during World War 2. I think we can call the Nazi and Stalinist art of the period Modernist, in the truest sense, since they were very ideologically motivated. After the war, art returned to post-modernism.