Complex sentence: Due to the illness of the regimental commander, Ashby had effective control of half of the regiment, which he operated separately, and when the commander retired in February 1862, Ashby assumed command of the entire regiment on March 12.
Split into simpler sentences: Due to the illness of the regimental commander, Ashby had effective control of half of the regiment, which he operated separately. When the commander retired in February 1862, Ashby assumed command of the entire regiment on March 12.