The people of South East Asia have quite a history of expansion as well as martial prowess in their home environment.
Remember the Kingdom of Champa? No?
Well, the Viets destroyed them very thoroughly and in fact, it was so bad that refugees flooded into Hainan Island.
Another warlike people who are often overlooked are the Burmese. During the Qing dynasty, there were three massive expeditions into Burma and all three ended in failure. People from other climates adapt very poorly to the humid insect infested environments. Gun powder gets wet, horses cannot be used and armour chafes while the heat and disease attacks Northerners very badly.
Dont underestimate the shirtless half dressed opponents with spears smeared with human faeces. The look that way because they have adapted very well and the faeces are very effective albeit very dirty - pun intended.
I agree with what you said, the last paragraph was very "descriptive", don't you think? lol.
I just wanted to add about Champa. The Kingdom of Champa was actually not an innocent state at all and they were probably the authors of their own demise through their constant warfare, leading to the exhaustion of their own man power. If you look into the History of the Kingdom of Champa (i'll provide you the link):
Champa - Wikipedia, the free encyclopedia
You will notice that Champa had two titular enemies: a) The Khmer Empire (Angkor) and 2) The Viets {Annam}. Early in Champa's existence, they feuded with Angkor , the principal legacy state of what was then the Kingdom of Chenla. The two, the Angkorian Khmers and the Champanese , fought constant wars with each other. Champa even sacked the capital of the Khmer Empire , Angkor, and then again in Lovek, multiple times. Then there were times that the Khmers sacked the capital of the Champanese in Indrapura. The constant warfare, depleted their man power , in fact this led to not only the exhaustion of Champa's population, but also the constant war put a strain on Khmer's population as well.
During the 8th to 10th centuries, there were 2 'foreigners' that were changing the environment of Southeast Asia:
a) The Tais (they would eventually make up Thailand and Laos) who came from what is now present day Yunnan, and 2) The Vietic Kinh who came down from Guanxi. The Tais and Vietic Kinhs were of course came with a large population ; the Tais defeated the Khmers and created their first Kingdom known as Sukkothai with their first king --- King Rhamkamhaeng the Great. Eventually, the Tais would subsume Khmer lands and culture , make it their own. As for Champa, it had a new enemy now, not the traditional enemy the khmers, but the Vietic Kinh. Champa , an indianized civilization, tried fighting the Viets in the traditional warfare style that it had implemented with the Khmers. This was basis for failure. Why? Because the Viets were Sinicized , and fought in Chinese-style military tactics , implementing horse drawn cavalry, utilized archers, utilized organized infantry blocks. Plus the Viet armies had utilized superior armor padding as comapred to the Champa soldiers who went to battle practically with just a wooden shield and a sword. They were easily dispatched.
Champa was defeated because for : 1) they had poor tactics, 2) they had depleted their man power through constant wars with the Khmers, 3) superior technology of the Sinicized Viet Kinhs.