Kmer had weakend themselves by vast building programme.
The construction of Angkor Wat during the reign of Suryavarman II (1113 – 1150)
The construction of Angkor Thom during the reign of Jayavarman VII (1181 – 1215)
Jayavarman VIII (1243 – 1295) was definitely a weak king. He could not intercept the expansion of Thai of the Thai dominion (kingdom).