What is uniquely American about it?
I know the history about how it was commissioned, and how Copland came up with the name. But because it was to honor those who were fighting in WWII, which includes American soldiers for sure, and also American workers who were busting their butts to provide the materiel for the war . . .
. . . but the effort wasn't Americans alone. The Allies, the partisan fighters underground . . .
. . . I've never thought of FFTCM as for Americans only . . .
. . . maybe that's what makes it the most American song ever?