September 1941 -- U.S. attacking German submarines

Some American sailors were actually seeing action a few weeks before
Pearl Harbor. And BEFORE September 1941, there was a very small number
of American civilians, who resided in Britain and Germany, who had
already volunteered to work or fight, for the country where they were
resident, although they were under no obligation to do so. FDR wasn't
the only American who chose his side before Pearl Harbor.