Complex sentence: This war was the continuation of the war of 1759--1760, the casus belli of which were over the control of the Tenasserim coast and its trade, and the Siamese support for ethnic Mon rebels of the fallen Restored Hanthawaddy Kingdom of Lower Burma.
Split into simpler sentences: This war was the continuation of the war of 1759--1760. The casus belli of the war were over the control of the Tenasserim coast and its trade, and the Siamese support for ethnic Mon rebels of the fallen Restored Hanthawaddy Kingdom of Lower Burma.