IN ITS predictable futility as much as in its 3,300 dead, wounded, and prisoners, the Dieppe raid was the worst disaster Canadian arms have so far suffered. Brian Villa wanted to know why it happened. The result is Unauthorized Action: Mountbatten and the Dieppe Raid (Oxford University Press, 315 pages, $24.95 cloth), a detailed, sometimes tendentious, almost always fascinating account of British military policy- making in 1942. Villa's villain is the handsome, dashing, rather brainless Lord Louis Mountbatten. But Lord Louis was not really the only one at fault. After all, Winston Churchill picked Mountbatten because he captivated Americans, raised him from naval captain to vice-admiral and used him to embarrass far abler but exhausted service chiefs. Each of them, as Villa makes Clear, had reason to support the hopeless Dieppe adventure in preference to even more harebrained and disastrous Churchillian schemes. Canadians went along because they had pestered the British for action and dared not back out, even when their ill-trained generals dimly realized the hopelessness of a landing without battleships and heavy bombers in support. Yet Mountbatten, eager to earn his new rank and glory, confident of his luck and soldiers' "pluck," made Dieppe happen. Frustrated by cancellation of earlier combined operations, Mountbatten never even got authority for the raid. His sole brilliance was managing his cover-up. The Canadian general became the agreed scapegoat; Mountbatten went on to greater glories.