Complex sentence: Fighting in the Hundred Years' War spilled from the French and Plantagenet lands into surrounding realms, including the dynastic conflict in Castile between Peter of Castile and Henry II of Castile.
Split into simpler sentences: Fighting in the Hundred Years' War often spilled from the French and Plantagenet lands into surrounding realms. This included the dynastic conflict in Castile between Peter of Castile and Henry II of Castile.