Complex sentence: Kesselring had the authority only to coordinate but not to prepare plans for combined operations by German and Italian forces and some influence on the use of the "Regia Aeronautica" for the protection of convoys to North Africa.
Split into simpler sentences: Kesselring had the authority only to coordinate but not to prepare plans for the joint employment of the German and Italian forces. He had some influence on the employment of the Italian air squadrons for the protection of convoys to North Africa.