Complex sentence: Force 136 was able to establish contact with Burmese communist groups, and through them with Aung San, commander of the Burma National Army (reorganised from the Burma Independence Army/Burma Defense Army).
Split into simpler sentences: Force 136 was able to establish contact with Burmese communist groups. Through the communist groups, Force 136 was able to make contact with Aung San, commander of the Burma National Army (reorganised from the Burma Independence Army/Burma Defense Army).