Complex sentence: They expanded their city-state into a thriving empire.Undeer Yikin Chan Kawiil, Tikal conquered Calakmul and the other cities around Tikal, forming what could be referred to as a super city-state.
Split into simpler sentences: They expanded their city-state into a thriving empire. Under Yikin Chan Kawiil, Tikal conquered Calakmul and the other cities around Tikal, forming what could be referred to as a super city-state.