You may not know, the Khmer Empire is the greatest empire in ASEAN history.
ថ្ងៃអង្គារ ៤រោច ខែស្រាពណ៍ ឆ្នាំខាល ចត្វាស័ក ព.ស.២៥៦៦ ត្រូវនឹងថ្ងៃទី១៦ ខែសីហា គ.ស.២០២២
Tuesday the 4th Waning Moon of Sāvaṇa B.E.2566 equivalent to August 16, A.D.2022 Year of the Tiger
ASEAN Urbanist

A Map of Southeast Asia, c. B.E.1444 A.D.900 by Digital Maps of the Ancient World

It shows the Khmer Empire in red, Champa in yellow and Haripunjaya in light Green, plus additional surrounding states.