Empires

Stele of Naram-Sin

Eventually the empire stops growing, sometimes because it meets neighbors who are stronger than it is, but more often just because it is inconvenient to fight so far from home.
After some time, the empire weakens and breaks up into smaller states again. There's really no need to explain why: it's hard to hold an empire together, and any little problem can make it fall apart.
The first empires were probably in Western Asia, first under the Akkadians, then under the Babylonians, and then under the Assyrians.The Egyptians also had a sort of empire in the New Kingdom as they conquered Israel, Jordan, Lebanon and Syria. China had empires a little later, under the Shang Dynasty.
The Hittites also had a small empire. These empires were mainly tribute-collectors, and did not try very hard to control the activities of the states under their control.

With the coming of the Persians in 539 BC, the West Asian empires got more centrally controlled, and the states that made them up got less independent. The Persian Empire was eventually taken over by Alexander the Great, but then broke up into a number of smaller states. After about 50 BC, Western Asia was dominated by the Parthian and then the Sassanian Empire and the Mediterranean Sea by the Roman Empire.
In the 400's AD, the Roman Empire broke down into smaller states again, and then in the 600's AD much of the Roman Empire and all of the Sassanian Empire were conquered by the Arabs and made into the Islamic Empire. The Islamic Empire, like other Western Asian empires before it, has a pattern of unifying and collapsing which has lasted from then until now. In Europe, no medieval power after Charlemagne succeeded in building a  successor to the Roman Empire, though many Holy Roman Emperors tried.

Chiefdoms
States
Examples of Empires:
Assyrians
Persians
Romans
Mongols
Islamic Empire
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