My hot take is that anti-fascists should sing the “Battle Hymn of the Republic” more, particularly at events in which they are tearing down confederate monuments, or otherwise protesting against neo-confederates (which is a huge chunk of fascists). Most Americans know the song, have strong emotions tied to the song, and it would immediately put the historical context of what you are doing firmly into place.
Plus, like I said, the song owns.
Counterpoint: We sing ‘John Brown’s Body’ instead, because it slaps just as hard with the added benefit of *not* idolizing the United States government or setting up the United States government as the embodiment of God’s will on Earth.
For though he’s gone to glory, still we struggle for the slave; and his soul is marching on.