The Khmer empire was the largest empire in South Asia for six centuries, ruling from the area around Angkor in today’s Northern Cambodia from 802 to 1431.
The early Khmer rulers adopted Indian culture and religions, brought by Indian traders.
A succession of kings marked their reign by building their state temple dedicated to a Hindu divinity, and later to Buddha. 
In addition to its military prowess, a key factor of the strength of the Khmer empire was the creation of a complex network of irrigation channels that made two or three harvests a year possible and could sustain a vast population.

The Khmer empire reached its apogee in the 12th century under its two great kings:

  Angkor Wat      

King Surya-varman II (1113-1150), the builder of the Hindu temple of Angkor Wat, and conqueror of most of South-East Asia.

 

 

    Bayon smile

Courtesy : www.pbase.com/wandertheglobe/cambodia

King Jaya-varman VII (1181-1218) who further expanded the empire and built numerous Buddhist temples, such as the Bayon and Ta Prohm. 

 

For more details, please visit Wikipedia’s Khmer Empire page