Complex sentence: The Confederate Army of the Potomac, whose name was short-lived, was the command under Brigadier General P.G.T. Beauregard, and whose only major combat action was the First Battle of Bull Run.
Split into simpler sentences: The Confederate Army of the Potomac, whose name was short-lived, was the command under Brigadier General P.G.T. Beauregard. Its only major combat action was the First Battle of Bull Run.