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What if the Allies had a plan to cut of the Axis retreat over the Straits of Messina? IOTL they really made no effort to do so, instead spending their air resources bombing the mainland and other parts of Sicily. That enabled 4 German divisions that went on to defend Italy, in fact making holding the mainland south of Rome possible, to escape: https://en.wikipedia.org/wiki/Allied_invasion_of_Sicily#Axis_evacuation
It was one of the most successful retreats of the war and one that could have been stopped. Axis AAA couldn't reach normal B-17 altitudes because German heavy AAA guns in theater that could were lost in or en route to Tunisia earlier in the year.
So what if the Allies had made sure to stop the evacuation before it began and the Axis couldn't evacuate most of their troops from Sicily?