In the nuclear age, transportation of nuclear weapons is pretty troublesome. The Russians have a lot of unaccounted nuclear bombs to look for after they had Ukraine, Belarus, and Kazakhstan transfer their weapons over to them after the fall of the USSR. The US has a lot of broken arrow incidents- accidents involving nuclear warheads that did not result in nuclear war. One of the most famous broken arrow incidents was the 1966 Palomares B-52 crash in which 4 hydrogen bombs were dropped near Palomares, Spain after the plane that carried them disintegrated when it collided with a fueling plane in midair.
Now let's assume the worst possible scenario. Let's say that the hydrogen bombs were armed at that time and at least one of them had gone off in the town of Palomeres. What is the impact of this throughout the whole world? Again, this TL starts in 1966.