We have seen how, by the ninth century, large parts of Asia and America witnessed the growth and expansion of great empires - some nomadic, some based on well-developed cities and trading networks that centred on them.
The difference between the Macedonian, Roman and Arab empires and the ones that preceded them (the Egyptian, Assyrian, Chinese, Mauryan) was that they covered greater areas of territory, and were continental or transcontinental in nature. The Mongol empire was similar.
Different cultural encounters were crucial to what took place. The arrival of empires was almost always sudden, but they were almost always the result of changes that had been taking place over a long time in the core of what would become an empire.
Traditions in world history could change in different ways. In western Europe during the period from the ninth to the seventeenth centuries, much that we connect with modern times evolved slowly - the development of scientific knowledge based on experiment rather than religious belief, serious thought about the organisation of government, with attention to the creation of civil services, parliaments and different codes of law, improvements in technology that was used in industry and agriculture. The consequences of these changes could be felt with great force outside Europe.