Warlord -
I suppose after 64 years it's easy to see many of the "screw ups" in the Italain campaign but the biggest to my mind was the total lack of a plan of any sort - whereas San Nazaire was very good plan -
one of the early screw ups was Clark's pulling the French back from near Altina to reinforce the failng 34th Division - had he reinforced the success of the French, then the tale might have been much different with thousands of casualties saved.
regarding the loss of the priceless monument - true many mosaics and frescoes could not be saved but - it appears that the Abbey is better than it was ...
have a look at this account ...
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BBC - WW2 People's War - The Abbey at Montecassino