Complex sentence: The local dynasty then regained local sovereignty and it remained semi-autonomous until 1931 when Nan finally accepted full Bangkok dominion, part of its territory had been annexed to Laos by the French in the late 19th century.
Split into simpler sentences: The local dynasty then regained local sovereignty and it remained semi-autonomous until 1931 when Nan finally accepted full Bangkok dominion. Part of its territory had been annexed to Laos by the French in the late-19th century.