Complex sentence: He was subsequently in command under William Rosecrans in West Virginia, and under Fremont in the Luray Valley, and after the battle of Cross Keys was, for a time, commander of the First Army Corps, in General Franz Sigel's absence.
Split into simpler sentences: He was subsequently in command under William Rosecrans in West Virginia, and under Fremont in the Luray Valley. He took part in the Shenandoah Valley Campaign of 1862, the Battle of Cross Key and was, for a time, commander of the First Army Corps, in General Franz Sigel's absence.