Complex sentence: He gained control of the entire Pomorje and the major maritime cities of the area, erected new settlements and started building a navy, but never succeeded in subjugating the lords of the independent Serbian statelets.
Split into simpler sentences: He gained control of the entire Pomorje and the major maritime cities of the area. He erected new settlements and started building a navy, but he never succeeded in subjugating the lords of the independent Serbian statelets.