Complex sentence: The Tikal-Calakmul Wars refer to a series of wars between Tikal and Calakmul, the First Tikal-Calakmul War being the first of these wars.
Split into simpler sentences: The Tikal-Calakmul Wars were a series of wars between Tikal and Calakmul. The First Tikal-Calakmul War was the first of these wars.