The key historical event that shaped the post-modern literary period was World War II and the revelation of the Jewish holocaust, or, if we want to broaden the scope, the whole period from 1914 to 1945, from the beginning of World War I through the rise of fascist dictatorships, the Depression, and World War II. That period unveiled horror after horror on the European continent. This naturally led Europe's leading intellectuals to start exploring what went wrong. How did the civilization—meaning the pan-European civilization that seemed so advanced, so civilized, and so happily on the path to using reason, rationalism, and science to build a better, more humane, and more beautiful world—end up at the holocaust and the mass destruction of World War II?
Post-modernism, which burst onto the scene in the 1960s, especially with Jaques Derrida's talk "Sign, Structure, and Play in the Discourse of the Human Sciences," could be seen as delayed or, more accurately, a slow-brewing response to the questions about human understanding that the barbarism of the first half of the century unleashed. Obviously, something was deeply wrong in how European thought was conceptualizing the world.
In Derrida's above-mentioned talk/essay, which you can easily find on line, Derrida argues that the framework we use to structure knowledge is simply a kluge, or what he calls a bricolage. We need support beams to uphold the structure of knowledge we're building, and for those support beams, we use whatever is at hand as long as it works. When it stops working and the edifice of knowledge begins to collapse, we stick in a new support beam. These bases of knowledge, however, are not necessarily Truth itself, just a rough approximation or guess. Therefore, we have to interrogate the structures of knowledge themselves. Derrida points particularly to the use of binary opposition: nature/nurture and male/female, for example, that have become ways of making sense of the world. Are they true? Or are they convenient kluges? Does this kind of "othering" when it comes to a binary such as "Aryan/Jew" lead to barbarism? It is important, most of the post-structuralists would insist, that we approach what we know, or think we know, with a great deal of humility because of the way we have misused knowledge in the past with tragic results.
I would say that the Cold War played a large role in how the Postmodern period evolved. For the Postmodern thinkers, the idea that World War II set out to end aggression and repression and then saw these same qualities represented by both sides in the Cold War helped to bring out the idea that there is a sense of nothingness in the world. In addition to this, I would say that the emergence of technology and the different and radical ways in which technology changed the way people interact with the world, with themselves, and with the concepts that define one's identity is another critical historical event that helped to mark the postmodern movement. Television, the advancement and decline of news print, the emergence of the internet, and the radical change in social networking are all elements that postmodern thinkers love to examine and play with from an intellectual point of view. This transformational nature also transformed art and what defines art for Postmodern thinkers. For Postmodernists, the 1960s was a time period that was critical because it was able to allow artists and thinkers to see what can be done from what is present, a basic tenet of intellectual expansion that is a part of Postmodernism.