In a word: Dieppe.

So, after Britain had been driven off the continent of Europe, she began to conduct cross-channel raids. Whether this was intentional or not, it resulted in getting in practice for the landings in North Africa, Sicily, Italy, and Europe (and figuring out what worked and what didn't).

Conventional wisdom was that for a successful invasion you needed to capture a port city due to the sheer amount of war materiel that needed to be moved ashore to support the invasion. So, Pas-de-Calais (a port city) would have been a good candidate for the invasion...in theory...until Dieppe on August 19, 1942.

The mission briefing for Dieppe was that the Canadian and British forces involved were to take the port city of Dieppe and hold it for a day. At least, that is what most of the Canadians and British soldiers were told. In reality, the raid was a cover for an attempt to attack the German navy's HQ in Dieppe and acquire intelligence material for the four-rotor Enigma machines that the U-Boats had started using (and Ultra had yet to decipher) - but that didn't come out until about ten years ago.

The raid, which involved over 200 ships and around 6,000 troops (of which around 5,000 were Canadians) was an unmitigated disaster. Around 1500 Canadian troops were killed or wounded, and another roughly 2000 were captured, basically annihilating an entire Canadian army (and, the British did not manage to acquire a four-rotor Enigma machine). British losses were comparable for the number of people involved. But, an important lesson was learned - trying to take a port city in the initial cross-channel invasion was a very bad idea, and it would be much better to simply build a harbour in place and then capture a port city after establishing the beachhead.

And that is precisely what the Allies proceeded to do at Normandy. The actual purpose of the Dieppe raid was hidden in cover stories, and the official history of the Royal Canadian Navy treated it as a place where tough yet valuable lessons were learned for future cross-channel landings.