Complex sentence: Campaigns like Edward I of England's wars in Wales, provided a good opportunity for a younger son of a landholder with no prospects in life other than becoming a monk or priest to become a mercenary soldier.
Split into simpler sentences: Campaigns like Edward I of England's wars in Wales, provided a good opportunity for a younger son of a landholder. With no prospects in life other than becoming a monk or priest, to become a mercenary soldier.