Empires
An empire is like a state, only with one more layer of government. States generally have at least four levels of administration; empires have five. An empire is generally made up of a lot of states joined together. Sometimes these states are still mostly independent, sometimes they are totally part of the empire. The usual pattern of empires is that one state, for whatever reason, becomes stronger than its neighbors and conquers them, creating an empire. The more it conquers, the stronger it gets, and so it conquers more and more of its neighbors. (There's also the victorious army problem).

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 400s AD, the Roman
Empire broke down into smaller states again, and then in the 600s
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.





