The secret to mass colonial conquest is political opportunism; exploiting divisions and weakness in the nations government in order to seize de facto control without ever really needing to actually literally conquer the country. Even when they fought wars the main goal was usually just siezing the capital/government (i.e. no need to take other cities/provinces etc). The result is that nations that managed to maintain strong and/or unified governments tended to resist far better than otherwise. Until eventually decades later a weak gov came along. In Thailands case they just never really had a poorly situated enough government to allow ready foreign takeover as well as being well situated in a buffer position between the french and english.
Burma is a great case that probably shouldn’t have been able to be conquered (it’s jungle terrain made operating in the country near impossible in the era) but a succession crisis led to an unpopular monarch and divided government. Seeing weakness the British pounced, declaring to the people they just wanted to replace the king, while also winning over (bribing?) the prime minister into getting the army not to resist.
They basically just went upriver in boats to the capital and the king surrendered to them out of hand.
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