Complex sentence: However, there is no contemporary evidence that he understood the importance of artillery firepower in modern warfare; instead in his writing at the time he criticised the "overrapid" offensive, the inadequacy of French generals, and the "slowness of the English troops".
Split into simpler sentences: However, there is no contemporary evidence that he understood the importance of artillery in modern warfare. Instead, in his writing at the time, he criticised the "overrapid" offensive, the inadequacy of French generals, and the "slowness of the English troops".