Complex sentence: The importance of the Belgian and French ports such as Calais and Boulogne-sur-Mer in supplying the BEF was perceived at the time: and in order to maintain an army in France at all, the allies had to control the English Channel.
Split into simpler sentences: The importance of the Belgian and French ports such as Calais and Boulogne-sur-Mer in supplying the BEF were perceived at the time. It is also true that in order to maintain an army in France at all, the allies had to control the English Channel.