In 1942, the Canadian main assault, called a reconnaissance in force, on the German-held village of Dieppe was a disaster. Of the five thousand Canadian infantrymen, fewer than half returned. In the air RAF losses against German airplanes was about two to one. All together, the Canadians, along with some hundred or so British commandos and American rangers, saw more than 1,000 men killed in action, and the engagement was concluded with a humiliating defeat. That's probably why you can find a dozen cinematic celebrations of Midway and D Day for every one of Dieppe. Movies, like history books, are written by the winners.
There were too many reasons for the failure to make them worth examining in detail. It was hastily planned, partly as a political move to show Stalin that the Western Allies were still in the war. And continuous postponements made it clear to German intelligence that a massive move across the channel was underway and that it's target was Dieppe. The result was that the defenders were well prepared.
A third reason is that communications between the commander, General Roberts, and the beach was quickly lost. German infantry picked out radio operators as primary targets. Ignorant of the failure of the assault, Roberts ordered more and more men onto the beach, long after it was hopeless. The same conditions held at Arnhem two years later when Polish paratroopers were dropped into an untenable cauldron of fire. Finally, the Churchill tanks that were to provide fire support were unable to travel across the gravel of the beach and promptly bogged down, making easy targets.
A common way of weaseling out of a mistake like this is to find some way of justifying it. Italy was probably a big mistake "but it tied down German troops that could have been used elsewhere." Right. It also tied down Allied troops that could have been used elsewhere.
The "accounting" -- in sociological lingo -- for this failure is that the lessons learned from it were successfully applied to the D Day landings two years later. In this case, however, it's a valid point. All of the dynamics of failure at Dieppe were avoided at Normandy, and the lives saved there were probably greater in number than those lost at the port of Dieppe.