The US started fighting U-Boats in the Atlantic before Pearl Harbor. It started Lend-Lease before Pearl Harbor. It started conscription to prepare for WW 2 before Pearl Harbor. It started sustained ground combat against the Germans in North Africa in November of 1942. Canada started in Sicily in July 1943. 

The "Americans came late to WW 2" is a Canadian myth and hides the fact that Canadian government policy was to avoid sustained ground combat for as long as it could, to have a navy consisting of small ships to avoid major casualties and to put a major effort into bombers (which were supposed to be a safe way to fight).