Complex sentence: England slipped back into the Second Barons'War, which was won by Henry's son, Prince Edward, who argued that the reformers had taken matters too far, and were themselves acting contrary to the terms of Magna Carta.
Split into simpler sentences: England slipped back into the Second Barons'War, which was won by Henry's son, Prince Edward. Edward argued that the reformers had taken matters too far and were themselves acting contrary to the terms of Magna Carta.