Trade and religion were a large part of the reason so many empires were formed and able to expand in this time period. In addition several other empires used conquering as a method of expansion. These methods were useful as both spread the influence of the empire.
Trade spread culture and religion throughout several regions. Because of this spread of culture and religion, people began to have things in common, which united them. For example, in Dar-al-Islam most of the empire was formed because of the spread of religion and culture through trade creating culture convergence in these regions. The method of warfare to expand empires led to large states. For example, the Mongols and most major European powers of the time.