They are distracted, now is our chance
Envoys from Tibet, the beloved Buddhist nations and good friend of the people of Khmer informed King Ang Chei I about a war for the freedom of the Deva Bengalese people. Assam had long tried to suppress the Bengal people in the Deva Bengal regions and eventually had angered it's neighbours. The Tibetan envoys asked King Ang Chei I whatever he wanted to join in but when he heard that Assam was allready losing he declined the offer.
But King Ang Chei I had other plans in mind. The only thing that had kept him attacking Shan had been Assam and now that Assam was busy taking over Shan would be easy. So King Ang Chei I prepared his armies and declared war on Shan. For every ones surprise Lan Xang decided to join Shan against Khmer. Assam joined too but their armies were never seen.
Lan Xang who hadn't paid attention to the allies of Khmer foolishly advanced over the border to the Central Thai territories. Even though the main army was occupying Shan territorry in the north the King Ang Chei I himself led another army guarding the home territories. Defeated by King Ang Chei I's armies Lan Xang armies retreated to their lands followed by the Khmers but the Lan Xang leaders soon found out that Dai Viet was all ready occupying their lands together with several units from Champa.
Soon the Lan Xang armies were defeated and the cities and towns conquered. The Kings of Lan Xang had no choice but to accept the subjugation by the Khmer.
So did the Shan.
So the Khmer Empire of the Old was slowly returning after the return of Shan territory and Lan Xang lands, even though not directly controlled the Khmers the shape of the Old Empire was forming. To secure peace in the region Ang Chei I allied the Burmese Kings.
A year later Ming decided to offer alliance to Khmer. Ang Chei I happily accepted it knowing that no one would mess with an ally of China.
Soon it became clear that Lan Xang as well as the other vassals needed to be fully incorporated in to the Empire. So Ang Chei I started to work his secret schemes with those loyal to Angkor.