Complex sentence: After the commencement of hostilities at Fort Sumter in April 1861, Major General George B. McClellan reentered the U. S. Army and, on May 13, assumed command of the Department of the Ohio, headquartered in Cincinnati, Ohio "War of the Rebellion".
Split into simpler sentences: After the commencement of hostilities at Fort Sumter in April 1861, Major General George B. McClellan reentered the U. S. Army. On May 13, he assumed command of the Department of the Ohio, headquartered in Cincinnati, Ohio "War of the Rebellion".