An era of territorial, political and commercial expansion followed the return to Angkor. Royal courts flourished and constructed several major monuments including Ta Keo, Banteay Srey, Baphuon, and West Baray. Kings of the period exercised their military muscle, including King Rajendravarman who led successful campaigns against the eastern enemy of Champa in the mid 10th century.
Just after the turn of the millennium, there was a 9-year period of political upheaval that ended when King Suryavarman I seized firm control in 1010CE. In the following decades, he led the Khmer to many important military victories including conquering the Mon Empire to the west (capturing much of the area of modern Thailand), thereby bringing the entire western portion of old Funan under Khmer control. A century later, King Suryavarman II led several successful campaigns against the Khmer’s traditional eastern enemy, Champa, in central and southern Vietnam.
Under Suryavarman II in the early 12th century, the empire was at its political/territorial apex. Appropriate to the greatness of the times, Suryavarman II produced Angkor’s most spectacular architectural creation, Angkor Wat, as well as other monuments such as Thommanon, Banteay Samre and Beng Melea. Angkor Wat was constructed as Suryavarman II’s state-temple and perhaps as his funerary temple. Extensive battle scenes from his campaigns against Champa are recorded in the superb bas-reliefs on the south wall of Angkor Wat.
By the late 12th century, rebellious states in the provinces, unsuccessful campaigns against the Vietnamese Tonkin, and internal conflicts all began to weaken the empire. In 1165, during a turbulent period when Khmer and Cham princes plotted and fought both together and against one another, a usurper named Tribhuvanadityavarman seized power at Angkor.
In 1177 the usurper was killed in one of the worst defeats suffered by the Khmers at the hands of the Cham. Champa, apparently in collusion with some Khmer factions, launched a sneak naval attack on Angkor. A Cham fleet sailed up the Tonle Sap River onto the great Tonle Sap Lake just south of the capital city. Naval and land battles ensued in which the city was assaulted, burned and occupied by the Cham. The south wall of Bayon displays bas-reliefs of a naval battle, but it is unclear whether it is a depiction of the battle of 1177 or some later battle.