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How would you manage to get the strong Siam empire as a simple European colony like most of Asia? Bonus point if it is not either France or UK.
Lastly as this is also a WI, what would be the effects of such scenario? You can count with 1 hand every single vaguely independent Asian nation, plus what would the post-colonial border look like? The scenario that I have in mind is a Thailand divided between British and French with the former having everything south of Bangkok and the northern Shan land and the French everything else; so the scenario I have in mind is an independent Shan state instead of OTL Burma plus bigger Lao and Cambodia. Malaysia would gain a little of the Southern Thai land and Thailand remains with nearly no minorities outside oversea Chinese.
The earliest POD possible is 1815.