Spengler becomes known for developing a theory about history moving in a cyclic fashion, that great civilizations have a natural and unalterable life cycle of rising, maturing, and eventually falling.
Oswald Spengler also argues that which cultures rise to become dominant world cultures is not governed by anything rational or obvious. This means there is no basis that would allow us to evaluate any particular culture as better or superior to others — there are simply more and less successful cultures out there, and it's the job of the historian to describe what happens to them.
At the time he wrote his material about the nature of history and human culture, he was be a believer in the importance of Germany in the development of Western culture. At the same time, however, he rejected the philosophy of the Nazis, who attempted to use his work for their own ends.