Complex sentence: The naval landings between Cannes and Toulon would be undertaken by the American 7th Army comprised of three American infantry divisions, with the Free French First Army landing after the beachhead had been secured.
Split into simpler sentences: The naval landings between Cannes and Toulon would be undertaken by the American 7th Army comprised of three American infantry divisions. The Free French First Army landed after the beachhead had been secured.