Complex sentence: Soon after the battle, the army was joined with Beauregard's Army of the Potomac to form the Army of Northern Virginia, which was led by Johnston until his wounding at the Battle of Seven Pines after which Robert E. Lee took command.
Split into simpler sentences: Soon after the battle, the army was joined with Beauregard's Army of the Potomac under the command of Johnston. Later the combined Army of the Potomac was renamed the Army of Northern Virginia, which was led by Johnston until his wounding at the Battle of Seven Pines after which Robert E. Lee took command.