Modernism broadly refers a set of beliefs that became dominant in the late 19th century and continued through most of the 20th century. These beliefs were generally that logic, science, and reason could help us learn from the mistakes of the past, and using what we learned, come to a deeper understanding of ourselves and of the meaning of human life. There is usually some sort of vibrant optimism in modernism, at least as far as the idea that if we just think hard enough and look deeply enough, we can make things better (at least understand things better).
Both World War I and World War II severely impacted the innate optimism in modernism. The general thinking was that we were progressing as a society away from ancient despotism, tribalism, and widespread warfare to a world where logic, reason, and economic interdependency would reign supreme.
The World Wars came and it wasn’t just “oh I guess we were wrong”…the world witnessed violence and genocide on a scale never previously seen. Whatever path we were on ushered in the worst horrors in history. And not only that, but the world wars had their epicenter in the SAME PLACE as all of this so-called progress (Western Europe).
Eventually the Post-modernists show up. As philosophers in these times tried to wrestle with this paradox, many of them came to the conclusion that this whole “history of progress” we were collectively writing was woefully misguided and naive, and many tried to “start from scratch” going all the way back to the basics: logic, reasoning, and rationality as guiding principles for a society.
They look at the core of all Modernist thought and say that objectivity was always a comforting lie. “All these ‘laws’ of yours are just stories you tell to explain the world to yourself. They might be useful, but stories change depending on the person telling them and the audience.” They got very interested in the idea that ideas can tell you about the people who hold them.
Modernism thinks human civilization can be perfected, but postmodernism is a lot more doubtful about this.
Modernism thinks that eternal concepts like truth and beauty can be investigated and defined if we work diligently, but postmodernism thinks this is a pointless exercise and mostly doubts that such things really exist at all, or at best are defined only temporarily.
Modernism is Star Trek. Postmodernism is Cloud Atlas.