The first real test of the Spitfire MkIX came with the Dieppe Raid of August 1942. This was designed to be a one day operation with the dual purpose of inflicting massive damage on the German-occupied port and of testing out the ideas about amphibious landings then taking shape. In essence it was a small scale trial for the invasion of France that would finally happen as D-Day on 6 June 1944.
The Dieppe Raid took place on 19 August and proved to be a disaster. Operatonal problems led to poor co-operation between the navy and army, while insufficient reconnaissance saw heavy attacks on unimportant targets while vital objectives were missed out altogether. Over half the mostly Canadian troops who landed became casualties and the tank force was wiped out.
In contrast to the Channel Dash fiasco, however, Fighter Command came out of the raid rather well. The new Spitfire MkIX was used in large numbers, as was the older Spitfire MkV. It was found that aircraft to aircraft, the Spitfire MkIX had the edge over the Fw190 above 23,000 feet, but that the German fighter was better below that altitude. But the efficient and effective rota employed by Fighter Command to send over Wing after Wing to a carefully co-ordinated timetable meant that fresh aircraft were always over Dieppe. The RAF won and maintained air supremacy over the town throughout the operation.
In the wake of the Dieppe Raid, Air Vice Marshal Trafford Leigh-Mallory was promoted to take command of Fighter Command. This had been his ambition for years and he threw himself into the task wholeheartedly. In November 1944 he was to be killed in an accidental air crash and replaced by Air Marshal Sir Roderic Hill.
from "Heroes of RAF Fighter Command in Sussex" by Rupert Matthews.