At http://www.eurekalert.org/pub_releases/2020-03/teia-dci032910.php tree rings from the Bidoup Nuibos National Park in Vietnam seem to show the fall of the Khmer civilisation coincided with drought – 600 years ago. The empire expanded through a large tract of SE Asia between the 9th and 14th centuries but two severe droughts, with heavy monsoon rains between, may have weakened the empire and it's irrigation system. In 1431 a raid from Siam is thought to have led to a reverse in fortune – and collapse. I'm not sure why they ignore the fact the Black Death occurred mid 14th century – perhaps it is thought not to have reached SE Asia. The plague may have led to a drop in population which may have meant the irrigation system was not maintained on the basis it may have required many humans to keep it in running order. However, in this study it is the tree rings that take precedence, shining a light on the period 1330-1360AD and another drought between 1400-1420AD. The drought is also recorded in China and Sri Lanka so was almost certainly associated with changes in the monsoon pattern. A similar sequence of drought and flood in the 1700s led to the collapse of powerful kingdoms in Vietnam and Myanmer.