M'hmm. Crude and very general (I'm tired; two young pussy-cats still don't quite like going nighty-night at reasonable hours
), but here goes.
1950s
Black civil rights movement and White Supremacist counter-movement inherently more radical/vioent (each feeding off the other). Several extremely nasty confrontations (take your pick as to which historical incidents are more viciously inflamed in this TL). By the end of the '50s, nothing has been achieved apart from tremendous distrust and radicalisation of both camps plus a good minority of the white population, hardened by bombings by black radicals, and otherwise moderate blacks outraged by increasingly hard-line government response.
1960s: Cuban Missile Crisis; Kenedy Assasination; very limited nuclear exchange
In the midst of the Cuban Missile Crisis, Kenedy is assasinated by a black radical Muslim extremist. The immediate and appalling result is an escalation to a limited nuclear exchange between U.S. forces and the Soviet commander in control of the new Cuban missile base. Three Soviet missiles are launched before Moscow can stop the exchange. One fails to detonate, a second engulfs a U.S. destroyer. But the third (City Buster) explodes over...? It should be a southern U.S. city; somewhere most likely further inflame black/white tentions (whites blame blacks not just for Kenedy's assassination, but for the resulting carnage). Charleston, perhaps? The result is a U.S. Nuclear response against Russian Cuban military base, a nuclear strike on Havanna, followed by a full invasion. Castro and his regime is
not treated kindly. Soviet prisoners also are tried as war criminals and executed out-of-hand. Very nearly a full U.S. Soviet nuclear exchange, but averted,
just.
1967: Six-Day War:
Already far more hard-line than our TL due to continuing domestic trouble, the U.S. is tremendously more generous in weapons deals with Israel. The result (encouraged by U.S. hard-line military) is an even greater disparity between Arab and Israeli forces.
1969: Madman Theory taken to it's logical conclusion
Nixon decides to use a single nuclear weapon as a terror-weapon against China. Peking is hit by a 1MT weapon. The Soviets are outraged, but stop
just short of a nuclear response.
1972: Munich Olympics: Israeli and U.S. athletes massacred
Due to U.S. greater and harder-line support for Israel and anti-Islamic rhetoric from severl hard-line U.S. politicians, Arab terrorists decide to target both Israeli and U.S. athletes. Made up entirely of Islamic extremists, they employ ISIS-style terror tactics, holding hostages and beheading athletes and also an American journalist who was caught in the siege. The result is a massacre
far worse than in OTL and a combined U.S. and Israeli response against Middle-East Arab military targets.
All right; was going to continue to Iran and the Iotola, but I'm simply too ready to go for a curl. Anyway, one should be able to extrapolate things just getting worse and worse from here.
Christian fundamentalist (or at least heavily influenced in that direction)? Shouldn't be a problem. Anti-Islamic. Again, definitely this should set the groundwork. Fascist? So long as things keep getting worse and everybody becomes ever-more radicalised; it should work. Anti-semitic? Probably impossible in this scenario. Unless of course you want to broaden the definition a little to include Arabs? Genocide? Ensure no hope of progress for black civil rights due to continuing domestic trouble and radical elements on both sides should keep growing more and more extreme until finally something truly spectacular blows up. After that: a military coup or hard-line right-wing elected government promising to suppress the troublemakers by
any means. And you have your African-American genocide, or at least something that well might lead to that being likely.
Far from perfect (probably border-line ASB to be honest), but I'm too tired at the moment to come up with something more plausible.