Why did Germany divide into four sections after World War 2?

The Third Reich was defeated by an alliance of primarily four nations, England, France, Russia, and the United States (with help from Canada and Australia). Each of those four allied powers was therefore able to assume control over one of four sections of Germany. The Russian sector became the state of East Germany, and the English, French, and American sectors were later joined to become West Germany. The two Germanies eventually reunified in 1990.