Complex sentence: Henry of Navarre and the House of Bourbon allied themselves to the Huguenots, adding wealth and holdings to the Protestant strength, which at its height grew to sixty fortified cities, and posed a serious threat to the Catholic crown and Paris over the next three decades.
Split into simpler sentences: Henry of Navarre and the House of Bourbon allied with the Huguenots, adding wealth and holdings to the Protestant strength. At its height, they controlled sixty fortified cities and posed a serious threat to the Catholic crown and Paris over the next three decades.