Complex sentence: Pope Innocent was one of the most powerful and influential popes in the history of the papacy, who exerted a wide influence over the Christian regimes of Europe, claiming supremacy over all of Europe's kings.
Split into simpler sentences: Pope Innocent was one of the most powerful and influential popes in the history of the papacy. He exerted a wide influence over the Christian regimes of Europe, indeed claiming supremacy over all of Europe's kings.