Basic story: GI Jackson (not real name) comes back from Vietnam to fight the KKKK - check the billboard (the extra K is for slightly Korny). Using the blacks in the community as an informant system, just like Stathis Kalyvas would suggest, the challengers take on "the MAN" (or MEN in this case) and kick some serious butt with funk - both tactically and musically. Campy but the brothers get busy cracking some crackers (had to do it). Oh, and yes, the brother in the poster is wearing a Klan outfit.
Film: US in origin (1976). Full film on Youtube (gotta love the net).