Complex sentence: The 45th regiment, under Colonel George von Amsberg, was closely engaged, and, with General Charles Devens now in command of 1st Division, bore the brunt of a concentrated confederate attack to the exposed Union right flank on.
Split into simpler sentences: The 45th regiment, under Colonel George von Amsberg, was assigned to 1st Division, General Charles Devens commanding. This division bore the brunt of a concentrated confederate attack to the exposed Union right flank on.