The short trip across the Strait of Dover afforded convenient garrison and supply by sea. However, the lack of natural inland defences necessitated the construction and maintenance of military fortifications, at some expense. Nevertheless, a critical factor in the stability of English government there over the centuries was the rivalry of France and Burgundy, both of which coveted the strategic position of the city; each left it to the English rather than to concede it to each other. Eventually, political strategies shifted at the division of Burgundian territory in the Low Countries between France and Spain and, when Henry VIII suffered setbacks in the Sieges of Boulogne (1544–1546), the approach to Calais opened to the south. Then in 1550 the Crown, in a crisis of royal succession, withdrew from Boulogne.