Complex sentence: England was ruled by King John, the third of the Angevin kings, but although the kingdom had a robust administrative system, the nature of government under the Angevin monarchs was ill-defined and uncertain.
Split into simpler sentences: England was ruled by King John, the third of the Angevin kings. Although the kingdom had a robust administrative system, the nature of government under the Angevin monarchs was ill-defined and uncertain.