Short answers, pulled from the story.
The Dieppe Raid, officially Operation Jubilee, was an Allied amphibious attack on the German-occupied port of Dieppe in northern France on the 19th of August 1942. Over 6,050 infantry, predominantly Canadian, supported by a Canadian tank regiment, were landed by a Royal Navy flotilla to test the feasibility of an opposed landing, gather intelligence, and demonstrate Allied commitment to opening a Western Front.
Of the roughly 5,000 Canadians who took part, 3,367 were killed, wounded, or taken prisoner, a casualty rate of 68 percent. The Royal Regiment of Canada alone lost 200 killed and 264 captured from a total strength of 556 men.
The raid failed because of a combination of faulty intelligence, the cancellation of preliminary bombardment, and alerted defences. British intelligence underestimated the German garrison at 1,500 men when the actual force numbered around 6,000, and had no knowledge of rail-mounted artillery hidden in tunnels in the headlands flanking the beach. The tanks were trapped on the shingle beach or blocked by obstacles in the town. A German coastal convoy encountered in the dark at 03:48 alerted the shore defences before the flanking landings could silence the batteries.
Dieppe demonstrated that preliminary artillery and aerial bombardment were essential before an opposed landing, that frontal assaults on defended ports should be avoided, and that accurate intelligence on enemy fortifications was critical. It also led to the development of Mulberry prefabricated harbours, the adaptation of tanks specifically for beach conditions, and the creation of an integrated tactical air force to support ground forces.
Louis Mountbatten was appointed chief of combined operations on the 4th of March 1942 at Winston Churchill's directive and held acting rank as vice-admiral, air marshal, and lieutenant general. He championed relaunching the operation after its first cancellation in July 1942, arguing the Germans would least expect a repeat attack on the same target. After the raid, his report to the chiefs of staff acquitted himself of responsibility and placed primary blame on General Bernard Montgomery.
RAF Flight Sergeant Jack Nissenthall was a radar specialist attached to the South Saskatchewan Regiment at Green beach with orders to infiltrate a German radar station near Pourville. Unable to enter the station, he crawled under fire to cut its telephone wires, forcing its operators to communicate by radio. Those transmissions were intercepted by Allied listening posts, providing intelligence about German radar capabilities along the Channel coast that helped develop radar jamming technology. His bodyguard unit carried orders to kill him rather than allow him to be captured.