Complex sentence: Force 163, planners for the operation, set the priorities for Dragoon the chief objective was the capture of the important French ports of Marseille and Toulon, which were considered as essential to supply the growing Allied forces in France.
Split into simpler sentences: Force 163, the planners for the operation, set the priorities for Dragoon; the capture of Marseille as a main supply harbor for the invasion. Initially, the chief objective was the capture of the important French ports of Marseille and Toulon, which were considered as essential to supply the growing Allied forces in France.