The Khmer Empire grew out of the kingdom of Chenla in what is now Cambodia around the 9th century AD and became one of the most powerful empires in Southeast Asia.
The empire is known to most people as the civilization that built Angkor, Cambodia's capital city.
The Khmer were an incredibly powerful and wealthy culture who were open to several belief systems including Hinduism, Mahayana Buddhism and Theravada Buddhism, which were the empire's official religions.
Their power also included military might as they fought many wars against the Annamese and Chams.
The decline of the Khmer Empire can be attributed to any combination of several factors.
The first being that the empire was ruled by a devarajo or god king, but with introduction of Theravada Buddhism, which teaches self enlightenment, the government was challenged.
This led to a lack of a desire to work for the devarajo which impacted the amount of food being produced.
But some scholars believe that these roads worked against them, making it easier for invaders like the Ayuthaya to get straight to Angkor.