FTP, name this ancient kingdom of Laos and Cambodia that lent its name to a 20th-century Cambodian communist group known as the "rouge".
After the founding of Ayuthia, the power of this empire declined.  This had already begun when their capital was sacked by the rival Chams' state.  Its golden age began with the rule of Yasovarman I, a descendant of Jayavarman II, who had united this empire.  It held sway over the lower Menam and left its mark with the magnificent temple complex of Angkor Wat.  FTP, name this ancient kingdom of Laos and Cambodia that lent its name to a 20th-century Cambodian communist group known as the "rouge".
After the founding of Ayuthia, the power of this empire declined.  
Khmer empire (prompt on early buzz of "Cambodians")