What is the main focus Oswald Spengler is trying to get across in The Decline of the West?

Spengler chooses several civilizations, or cultures, and proceeds to show how each was born, developed, declined and fell. Essentially, the overall pattern is that cultures are born and develop in agrarian societies, which then become gradually urbanized. The wealth and sophistication that comes with urbanization leads to corruption, which is what causes a civilization to fall. His central, and in context, the most disturbing point, is that the west, having reached its peak in the late nineteenth century, will decline and fall throughout the twentieth century, to be replaced with a dictatorship that will reject many of the liberal advances that western culture held dear.