Complex sentence: Upon Hancock's return from medical leave, and the spring 1864 reorganization of the Army of the Potomac, Warren assumed command of the V Corps and led it through the Overland Campaign, the Siege of Petersburg, and the Appomattox Campaign.
Split into simpler sentences: Upon Hancock's return from medical leave, and the spring 1864 reorganization of the Army of the Potomac, Warren assumed command of V Corps. He led the V Corps through the Overland Campaign, the Siege of Petersburg, and the Appomattox Campaign.