Complex sentence: In this work he for the first time showed the connection between the internal and external history of France; he was also the first, by a systematic study of the records, to check and correct the traditional account of many episodes in the internal history.
Split into simpler sentences: In this work he showed for the first time the connection between the internal and external history of France. By systematically studying the records, he was also the first to check and correct the traditional account of many episodes of France's internal history.