If you aren’t American: Depicted are American civil war reenactors with the flag of the Confederacy which seceded from the Union of the United States in the 19th century due to built up tensions regarding slavery. The Confederacy were ultimately fighting to keep slavery as a government institution.

The target of the joke are tankies (described briskly above), which we will see on Lemmy, often see any action taken against the US or the EU (the West) as praiseworthy, blatantly disregarding or downplaying equivalent or worse actions from non-western nations.

The subversion of expectation here is that a tankie would see the pro-slavery Confederacy as praiseworthy because they’re fighting the US government, despite the Confederacy’s blatantly immoral cause.

Caveat - I say this with bias as someone explicitly against authoritarian bootlickers, like tankies.