Kim-Jong Il decided to try and build a nuclear doomsday device called the Glorious People's Revolutionary Hammer to threaten the west into handing over Japan and Manchuria, since they have Korean populations, obviously they are all part of Korea as well. The rest of the world responded with a nuclear carpet bombing, turning Korea into a wasteland.
Vicksburg, Jackson, and all surrounding communities were turned into this timeline's version of Pripyat when a shoddily-made mass-production nuclear facility, during a routine start-up test, malfunctioned and suffered a meltdown.
The US is kind of in the shitter at this point, with racial tensions at an all-time high, militias everywhere, a flu pandemic, an energy crisis exacerbated by said nuclear meltdown, and the major 1980 presidential candidates being California Governor Charles Manson and Pennsylvania Governor Jim Jones (yes, seriously). The Commonwealth is doing okay-ish for the most part, although things are a bit awkward now that England voted to leave the U.K. The USSR is pretty much the world's sole superpower, even after the Sino-Soviet War, although the man in charge now is none other than Andrei Chikatilo.
They'd formed a NATO-style defense pact called the Jerusalem League in response to Soviet aggression early on, and were doing mostly okay up until 1975. Chikatilo ordered nuclear strikes on many of the major cities, as well as the High Nasser Dam, to destabilize the region and ensure the USSR gained a monopoly on the world's oil supply. The region is now not doing well.
Probably a few million, at least.