In the 19th century the Great Powers were the larger European states: Britain, France, the German Empire, Austria-Hungary and the largest, Russia.

A Power is a state capable of exerting its will on others.

In this sense there have been only two Powers since 1945: United States and the Soviet Union. Only these could exert military power in every part of the planet.

From the fall of the Soviet Union only the United States has this power. There are however other financial and economic Powers: Germany, Japan. If economic power is now more important than military power, perhaps these are the superpowers, alongside the United States. Can other powers arise? Perhaps China may rival them, especially if it continues its economic growth.

The wars in Vietnam and Afghanistan showed that even a Superpower can be defeated by high enough morale.