This may not be a popular opinion but...
The fight against the darkness that the Axis represented predates Pearl Harbor. There were even eleven unofficially-known-to-be American fighter pilots in the Battle of Britain.
The exact number is difficult to determine as they had to pretend to be Canadian to avoid potentially being stripped of their citizenship and imprisoned under US neutrality laws, but if they passed beyond the juristiction of any earthly law court (of the eleven seven were KIA and one DoW) it was usually acknowledged.
A case in point was William Meade Lindsley Fiske III, whose claim of being Canadian did somewhat stretch the bounds of credibility (he'd won two Olympic gold medals for the US, and carried the Stars and Stripes in the opening ceremony of the 1932 Winter Olympics, which was held
in the US). He volunteered and flew with 601 sqn until his plane was shot up. Even then he managed to land it, despite the fact he was wounded and the plane was on fire. He died of his wounds two days later.
His coffin, carried by six members of his ground crew, was draped in the Union Jack and the Stars and Stripes.
There's a plaque commemorating his sacrifice in St Paul's Cathedral, and the church where he is buried now has this window: