I don't suppose I'm saying anything people here don't already know, but for the general public who wander onto this site through google searches there might be misconceptions. A very large number of fine young men from America joined the RCAF long before Pearl Harbour. They were trained under the BCATP, got their RCAF wings, then served with the RCAF and often were posted to RAF squadrons for years before the USAAC called them.
See this photo of Godfrey and Gentile. Notice they're wearing 2 sets of wings.

They were tremendously proud of having taken this path, when the US was still largely gripped by isolationism, and attitudes exemplified by the "America First" movement supported by famous people such as Lindbergh and Henry Ford. Hence both wings. Some fit into their RAF world so well that they thought long and hard about leaving.
Lance Wade was one such -- already an ace before Pearl Harbour, before the Flying Tigers. He never did leave the RAF. Kept refusing. Ended up an RAF Wing Commander with 25 kills in the MTO.
People like these were extremely useful when the US came into the war, because they were current, and blooded. The training and experience they'd received in the RCAF and RAF -- actual operational experience -- was invaluable, and of course was why they were recruited so heavily into the USAAC.
Another fine young American went this route, John Gillespie Magee, whose poem is in nearly every flight ops room in the world.

Dave