Complex sentence: Under the rule of the country's first King, Croatia became one of the most powerful kingdoms in the Balkans; nobles possessed unprecedented power over the governed, and were one of the first nobles to advocate for monarchical absolutism.
Split into simpler sentences: Under the rule of the country's first King, Croatia became one of the most powerful kingdoms in the Balkans. Nobles possessed unprecedented power over the governed, and were one of the first members of royalty to advocate for monarchical absolutism.