OTHER CIVILIZATIONS
Babylonia
18th Century
Babylonia was an ancient civilization much like the Khmer empire. They spoke Akkadian and were a Semitic state and cultural region founded in central Mesopotamia (what we know today as Iraq). A small Amorite-ruled state emerged in 1894 BC, which contained the city of Babylon. Babylon greatly expanded during the reign of their king Hammurabi in the first half of the 18th century BC. While the growth of the empire was massive they used up too much natural resources like rivers and forests leaving their territory without any natural barriers. After that it was only a matter of time until they were attacked by neighboring empires.
The Roman Epire
(31 BC) - (476 CE)
The Roman Empire was the post-Roman Republic period of the ancient Roman civilization. The 500-year-old republic which came after it was severely destabilized in a sequence of civil wars and political conflict, during the time when Julius Caesar was appointed as the dictator and then dramatically assassinated in 44 BC. A period of increasing decline began with the dictator Commodus. Commodus' was assassinated in 192.
Then there was another civil war that was finally ended by Constantine I. Then the Roman Empire was renamed Constantinople in his honor. It remained the capital of the east until its demise.
Summary
It seems that in this particular case the officials and dictators were too busy arguing to notice the decline and the fall of the empire and was crippled because of it. Their neighbors invaded then soon after like the Babylon Empire they were their own worst enemy.