Complex sentence: Relieved by the long confrontation between Burgundy and France, English rule over Calais was able to flourish for 150 years, these two neighbours each coveting the city but preferring to see it under the English rather than their rival.
Split into simpler sentences: Relieved by the long confrontation between Burgundy and France, English rule over Calais was able to flourish for 150 years. The French and the Burgundians each coveted the city, but preferred to see it under the English rather than their rival.