The Funan Kingdom, believed to have started around the first century BC, is the first known kingdom of Cambodia. The kingdom was strongly influenced by Indian culture by shaping the culture, art and political system.
During the 9th century, King Jayavarman II started a cult that honored Shiva, a Hindu god, as a devaraja (god-king) which then linked the king to Shiva. He also began the great achievements in architecture and sculpture while his successors built an immense irrigation system around Angkor.
By the 12th century, Cambodia had spread into other areas, now known as Thailand, Laos, Myanmar and Malaysia (the peninsula). There is actually still evidence of Khmer inhabitance in Thailand and Laos to this day.
The 13th and 14th centuries were not as successful for Cambodia, some believe it was due to the increased power of (and wars with) Thai kingdoms that had at one time paid homage to Angkor.