The problmes with Dieppe are numerous
1. As stated above, the motivation was political and therefore militarily suspect to begin with. It has to be said though that it was pressure from the US as well as the USSR that lead to the idea of a raid or as it is sometimes stated a 'reconnaisance in force'
2. The planning was appalling. Early on the objectives were established, mostly in a rather arbritrary 'wish-list' fashion. As the operation was planned and abandoned twice before jubilee, but the objectives were never reappraised, it lead to the methods and tactics becoming divorced from the objectives they were supposed to achieve
3. Despite the operation officially coming under the office of 'Combined Operations', CO was in itself a new concept, and as such did not have the full support of all branches of the British military establishment. The Navy in particular resented the fact that it was being planned by Mountbatten (whose highest previous position had been as captain of a ship) and resented the fact that it was not, as an amphibious operation, a 'Navy show'. There is some evidence that they did not believe in or support the raid and witheld valuable naval assets that could have contributed to the effectiveness of the operation, in particular with a preparatory bombardment (something that was to be highly effective 2 years later in Normandy) there is also an argument that these assets were simply too precious to risk considering the need to defend British waters
In addition the RAF provided only fighter cover. Many people agree that bomber support should have been used in the raid. The RAF was to suffer heavy losses during the raid (a fact often overlooked but very evident in the Cemetary at Dieppe) when their Spitfires faced the more advanced FW190s for the first time
4. Nobody it seems wanted to be the ones to say the raid was untennable. The Canadian Divisional Commander, senior Naval Commander and Lord Mountbatten all had the oportunity to 'pull the plug', or at the very least register their disagreement with the plan. Unfortunately none did
As for the main objectives, from memory they were:
1. To seize and disable the batteries at Puis (achieved)
2. To seize and disable the batteries at Pourville (not achieved when commandos became pinned down)
3. To seize the town of Dieppe via frontal assault (Infantry objective not achieved)
4. To push through Dieppe and rendezvous at a point beyond the town (Tanks objective not achieved largely due to the tank blockades in the narrow streets leading off the promenade)
5. To seize German infantry barges moored in the docks at Dieppe (commando objective not achieved when the landing craft too heavy fire from the mole at the mouth of the docks) It is also doubtful that the barges could have made it back across the channel anyway
As mentioned above, several lessons were learned at Dieppe.
Chief amongst these were:
1. the necessity for specialised armour to overcome beach objectives while offering relative safety to engineers (the engineers at Dieppe were infantry and were all killed or pinned down on the beach)
2. The need to a heavy, sustained and targeted naval bombardment in amphibious assaults
3. the need for local air superiority over the landing area
I should say, this is my understanding based on things I've read. This is a subject much open to interpretation, and well worthy of study for anyone interested in combined or amphibious assault operations.
Chris