Why Axis command could not become unified

Why was a unified Axis command structure never really possible?

Axis political relationships and command culture were far less cooperative than Allied arrangements, even where Italian commanders were capable.

Allied arrangements allowed officers to serve across national forces and make working relationships function. Cross-posting and practical accommodation gave the Allies habits of cooperation that the Axis powers lacked. In North Africa, capable Italian commanders could still remain subordinate while Rommel retained command for political reasons. Alliance politics could therefore override a rational division of military responsibility.