Complex sentence: The Union advance stalled when they encountered the Confederates on the opposite side of a muddy marsh where they blazed away at one another for more than two hours with musket and cannon fire, until the arrival of Confederate reinforcements.
Split into simpler sentences: The Union advance stalled when they encountered the Confederates on the opposite side of a muddy marsh. The opposing forces blazed away at one another for more than two hours with musket and cannon fire, until the arrival of Confederate reinforcements.