Complex sentence: He had long been an ally of the Thai against the Burmese; however, due to lack of recognition for his accomplishments, from 1826 to 1828 he rebelled against Siam in an attempt to regain complete independence.
Split into simpler sentences: He had long been an ally of the Thais in their wars against the Burmese. However, during the period between 1826 and 1828 he rebelled against Siam in an attempt to regain complete independence.