Final answer:
The arrival of the Thai kingdom in the north and environmental challenges contributed to the decline of Angkor.
Step-by-step explanation:
The decline of Angkor can be attributed to multiple factors. One key factor was the arrival of Thai kingdom in the north. The Thai kingdom took control of Angkor in 1431, effectively ending the Khmer empire. Additionally, the Khmer empire faced environmental challenges with their water system, leading to compromised flood control and the filling of the water system with silt. These factors, along with invasions from the Mongols and the formation of other kingdoms in the region, contributed to the decline of Angkor.