I'm sure it's been discussed at length elsewhere many times before but Mid-World, where Roland is from, could have been an alternative world in which The Stand took place and Flagg's plans succeeded in one way or another. The evidence being that some form of cataclysmic event occured to the severity that the entire world had to start over. It's perfectly possible that in that level and time, Flagg WAS able to fire off nuclear weapons. Just think about the vast wastelands, the mutants, the technology which was lost to time under rubble and seemingly where people took shelter underground.
Then again it's all one big loop so it's fair to assume whatever you want. Flagg could have destroyed the world, someone could have risen against him and temporarily defeated him, and the world just 'moved on'. Flagg may succeed temporarily in endeavours but the White always catches up to him in the end.