Champa was a kingdom in OTL South Vietnam.
1203, the Khmer ruler Jayavarman VIII occupied Champa and made it a Khmer province. As soon as 1220 however, Champa regained its independence.
1310, the Vietnamese king Tran Anh Tong defeated Champa and made it a vassal. But when Annam became a tributary of Hong China in 1376 (3073, Yang Fire Dragon), Champa threw off Annam's influence. 1385 however, Hong China and Annam made Champa a tributary again.
1668, the Khmer ruler demanded that his country should get overlordship over Champa and Thai lands. And 1707, Champa stopped paying tribute to Hong, with impunity. The example was soon followed.
Arrival of the Europeans
But in 1766, the Indochinese League fell apart, when Khmer conquer Champa and France, caught in the republican troubles, couldn't interfere.
1780s: Champa, Thailand and Pegu (S Burma) invade the sinicized nations of Lan Xang (Laos), Annam and Ava (N Burma) and even the South of China, including the former kingdom of Dali. It is quite obvious that the emperor has lost the Mandate of Heaven.
1792: Italy made a trading treaty with Champa. After 1805, Italy supported them with arms against China.