Preliminary comment: This post focuses on Thailand and especially Burma more than Vietnam, which has different military traditions from most of Southeast Asia due to being part of the East Asian cultural sphere.

16th-century armies in Mainland Southeast Asia impressed Europeans with their grandeur. As one Venetian said in 1569 about the King of Burma with characteristic hyperbole:

There is not a King on the Earth that hath more power or strength then this King of Pegu [the capital of Burma], because hee hath twenty and sixe crowned Kings at his command. Hee can make in his Campe a million and an halfe of men of warre....

This King of Pegu hath not any Armie or power by Sea, but in the Land, for People, Dominions, Gold and Silver, he farre exceeds the power of the great Turke [the Ottoman sultan] in treasure and strength. This King hath divers Magasons full of treasure, as Gold, and Silver, and every day he encreaseth it more and more, and it is never diminished.

Early Modern Mainland Southeast Asia was a land fraught by war: the great Burmese invasions of Thailand, the endless civil wars in Cambodia, the epic battles between North and South Vietnams. Naturally, warfare was a dynamic and innovating process throughout the era under discussion.

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What were armies like?

Let's talk about size first. Some claim that local armies were of historically unprecedented size. Remember how our Venetian talked about "a million and an halfe men of warre." Alternately, consider the report of the 16th-century chronicle Nidana Ramadhipati-katha from what is now Burma, describing the exploits of the Burmese king Bayín-naung ("the King of Pegu" in the Venetian's account).

[in 1563] the King gave the word to march on Ayudhaya [capital of Thailand]. His forces at this time, not including the Chiengmai rebels, amounted to more than 900,000 men [sic!], with 500 tuskers and 4,000 horses...

Clearly, armies this big are simply impossible. The population of the entire country of Burma at the time was less than two million, for one thing. An army of 900,000 troops would be equivalent to conscripting the entire adult population, both men and women.

So why this level of exaggeration? To answer this question, we need to look at the biases of Southeast Asian historians. The historians who were giving such numbers were very often high aristocrats and military commanders themselves and would have immediately known that million-men armies were impossible. Instead, such hyperbolic numbers reflect what the writers at the time thought of the ruler. To use Burmese terminology, a king like Bayín-naung could attract a million men to his cause only because his good karma gave him much hpòn. Hpòn is a sort of spiritual power and authority that attracts a clientele, and with more hpòn the more numerous one's followers. Hence saying that Bayín-naung had an implausibly large army simply implies that his hpòn was unprecedentedly great, and that he was that good of a ruler.

(European accounts are even more hyperbolic, talking about armies of two million troops and such, but for slightly different reasons. One big one is glorifying European successes and handwaving away defeats by exaggerating how large their enemies were and by ignoring European reliance on local allies. Another one is to emphasize the earthly riches of Southeast Asia and potentially attract freebooters.)

If local sources' estimates of army sizes are actually the writer's comments on the political situation more so than an actual estimate of the number of soldiers, how big were actual armies? In the 1760s, we are reasonably sure that the Burmese king Alaùng-hpàyà led 50,000 troops against Thailand. Alaùng-hpàyà was a masterful and charismatic general who would naturally be able to raise larger armies than most normal kings. Even so, this force was around 10% of the Burmese adult male population and heavily strained the kingdom's resources. So we can posit 50,000 as the upper limit for Burmese armies. Thailand's population and political situation were similar, so we can reasonably infer that it could not have raised much more than 50,000 either.

Just to talk a little about Vietnam, armies there were rather larger. In the 17th century, the country was split in half between the dictatorships of the Trịnh family to the north and the Nguyễn family to the south. It's likely that the Trịnh could raise 80,000 troops when they invaded the Nguyễn domains, and this larger army reflects first and foremost the facts that northern Vietnam was the most densely populated area of Southeast Asia and that Vietnam itself had by far the most people.

Still, 50,000 is an impressively large army. How were these people raised?

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Were they mostly peasant levies or professional troops?

Most troops were peasant levies. To quote one Persian observer who visited Thailand in the 1680s:

It should be mentioned that in all the countries of the Lands Below the Winds [the Persian name for Southeast Asia] the peasantry and the army are one and the same and if the army suffers the country itself falls into ruin.

In Burma, the king's commoner subjects were divided into two groups: the ahmu-dan and the athi. The former worked for the government, including as soldiers, while the athi paid taxes instead. The central government established a quota of levies for each township based on estimated population. Each township governor received this quota and set sub-quotas for the ahmu-dan villages to fill it, and the actual people who would be conscripted was decided on the village level.

The ahmu-dan villages were, however, a military organization onto itself. An ahmu-dan village might be considered part of the Shway-pyi Ya-naung Cavalry Regiment, for example. This village was inhabited by soldiers and ex-soldiers of the Shway-pyi Ya-naung Cavalry Regiment and their wives and children. Supplying a certain number of men to serve as Shway-pyi Ya-naung Cavalrymen would be the hereditary obligation this village owed to their king. Everyone in the village would know that they were cavalrymen in some sense and find pride in their position. The village chief would always be the cavalry officer, and the township governor in this area would always be the commander of the Shway-pyi Ya-naung Cavalry Regiment.

So while the troops were peasant levies, it wasn't as if farmers were being given a spear out of the blue and told to fight. Villages were organized along military lines - this village over here is inhabited by the Marabin Artillery, that one over there belongs to the Shan Cavalry - and there was a degree of professionalism.

Similar dynamics prevailed in Thailand, the main difference being that the Thai analogue to the ahmu-dan (the phrai luang) had much less prestige in Thai society than in Burma. The Thai kings literally tattooed the phrai luang to mark them as slaves of the court, while the ahmu-dan in Burma were, as loyal servants of the king, probably the most honored class of commoners.

Vietnam originally had a similar system, but transitioned earlier towards a standing professional army. This again reflects Vietnam's greater economic and demographic resources, as well as the influence of the Chinese tradition of bureaucracy.

There were professional troops. As mentioned above, Vietnam developed a standing army. In Burma and Thailand these included mercenaries from around the world: Europe, the Middle East, India, Indonesia, Japan. They were a double-edged sword, though, since they could and did interfere with government policies or even revolt at times. In the early 17th century, for instance, the king of Thailand struggled to control his cohort of Japanese mercenary samurais. There were also standing royal guards who were heavily disciplined. Even they were a potential threat to royal authority, which is why both Burma and Thailand ended up having to regularly rotate the members of their guards so they wouldn't get caught up in factional politics.

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How were they armed?

The broadsword was the most common weapon, perhaps because it was a familiar tool for most Southeast Asian peasant levies. They used swords as machetes to cut down trees and bamboo, and it was the basic weapon of self-defense in the tiger-infested jungles of the region.

Other weapons included lances, spears, and pikes, all valuable for both infantry and cavalry. The bow and arrow were important projectile and incendiary weapons. Crossbows were also used throughout the period you're talking about. However, at least in Burma blowpipes were quickly displaced by guns. This might be because the lethality and shock factor of the blowpipe was much less than either the bow or the gun (in places in Indonesia where poison darts were used, the blowpipe took much longer to be abandoned).

In terms of armor, leather helmets and leather shields were used. Body armor depended. It was common for soldiers to charge into battle wearing only a loincloth, but in the 17th century the soldiers of Thailand wore thick cotton armor. The elite, of course, were different. Vietnamese officers alone wore chainmail, while one Burmese chronicle describes the following scene:

[Bayín-naung dressed in] his excellent nine-jewelled chest chains, and a sash suspended from his left shoulder, in the style of the leading kings of the Raman country when dressed for great victory on the battlefield.

To sum up, this European drawing from 1800 of a common Burmese infantrymen is probably typical.

Continued below.