The raid on Dieppe, Belgium (also known as the Battle of Dieppe), on August 19, 1942, was one of the most disastrous battles undertaken by the Allies during WWII, perhaps second only to the disaster of Operation Market Garden, an airborne assault to capture bridges over the Rhine that suffered from poor planning and execution.
First, because the Russia was being attacked by Germany at this time and urgently needed the Allies to distract the German Army, Russia pleaded with the Allies (chiefly, Britain) to provide some sort of action that would require Germany to fight on a front other than the Russian Front. Second, Canada had recently supplied thousands of troops who were not yet engaged in any fighting, and the Canadian government put pressure on Britain to put these troops to use, preferably fighting Germans.
The British believed Dieppe was a good target for a beach assault because it was (they believed) lightly defended. The first assault was to have been made primarily by about 5,000 Canadian troops, about 1,000 British paratroopers (who were going to provide a diversion) and a few U. S. Army Rangers. In addition, just before the actual assault, there was to be a massive aerial bombardment of the area around Dieppe. Unfortunately, the aerial bombardment was drastically reduced in scope; the diversion to be provided by British paratroopers was cancelled; and intelligence efforts to determine the size of the German army protecting the Dieppe area was negligible (the British used photos taken by vacationers before the war to plan the assault). The British didn't even know, for example, how much armor (tanks and large artillery pieces) the Germans had available in the Dieppe area
On the morning of the main assault, the element of surprise was lost when German units and British ships protecting the landing craft exchanged gunfire, thereby alerting the Germans that an attack at or near Dieppe was imminent. When the Canadian units came ashore at Dieppe, the Germans essentially wiped out most of the Canadian troops as they reached the beaches near Dieppe--very few troops actually made it into Dieppe.
Even though the Canadian troops who reached shore fought very well, they were outnumbered and outgunned and had no chance of actually over-running the German troops in order to capture Dieppe. Between about 11:00 a. m. and 2:00 p. m., the Canadian forces withdrew from the beach back to their landing craft and out to sea, but not before leaving almost 3,500 dead, wounded, or prisoners in the hands of the Germans. The Royal Air Force, which had provided some limited air cover for the operation, lost about 110 aircraft to the Germans, who lost fewer than 50. German Army casualties were less than 600.
The only redeeming aspect of this battle was that it taught the British and Americans how not to mount an amphibious raid on a well-protected position. These lessons, learned with the loss of so many good troops, eventually helped the Allies plan the invasion of Normandy. One could argue that without the disaster of Dieppe, the Normandy invasion might have been unsuccessful, but the fact remains that the Dieppe raid was the result of poor planning and faulty assumptions on the part of the British high command. As is often the case, the troops fought well but had no real chance of success.