A completely different direction of thought:

Maybe in that future, nuclear power was utilized so widely and intensively that all uranium has already been used up. Since the world had gone through a peaceful phase, this also included any uranium originally used for nuclear bombs, as well as any plutonium that could be generated (for a while, plutonium power plants were used to replace the uranium plants running out of fuel). There are no nukes, for lack of fissile material.

Since it was finally figured out how to build viable nuclear fusion plants just when the last stocks of fissile materials got used up, this did not lead to an energy crisis. But when hostility returned to the world, nobody could figure out how to detonate a thermonuclear bomb without a fission starter.