Complex sentence: By this time, General Ulysses S. Grant had taken over Halleck's responsibilities in the Western Theatre, and the 8th Wisconsin was still part of the Army of the Mississippi, but General William Rosecrans had replaced Pope as commander.
Split into simpler sentences: By this time, General Ulysses S. Grant had taken over Halleck's responsibilities in the Western Theatre. The 8th Wisconsin was still part of the Army of the Mississippi, but General William Rosecrans had replaced Pope as commander.