The military logic for Sicily was sound. Capturing the island would clear the central Mediterranean sea lanes and allow Allied shipping to transit without the long detour around Africa. It would remove the island as an air base for Axis attacks on Allied Mediterranean convoys. And it would put Allied forces within striking distance of mainland Italy, potentially knocking a weakened Italian state out of the war entirely.
But Sicily was a large, mountainous island defended by both Italian and German forces. The obvious nature of the target was itself a problem. As General Sir Alan Brooke, the Chief of the Imperial General Staff, wrote in his diary following the Casablanca decision, there was a genuine concern that German and Italian planning would anticipate the attack. An island that was geographically obvious as the next step after North Africa would be an island that the enemy would prepare to defend.
General Freddie Morgan, working for what would become COSSAC (Chief of Staff to the Supreme Allied Commander), described the dilemma succinctly in later accounts: the Allies had to attack somewhere, the enemy knew they would attack somewhere, and the geographic logic of the situation pointed toward a limited number of targets. Any one of those targets would receive reinforcement if it could be made to appear the most likely choice. The question was whether anything could be done to shift that apparent likelihood.
The German commander in the Mediterranean theater was Field Marshal Albert Kesselring. Kesselring was a capable and experienced officer who understood geography. His assessment, without any deception operation, was that Sicily was the most probable Allied target. German intelligence was already producing reports suggesting Sicily. Hitler himself, according to postwar accounts of his deliberations, occasionally mentioned Sicily as an obvious Allied objective.