The Atomic bomb became the Battleship of the modern day, too pricey for most, too important for the few, seemingly indispensable for war yet too risk to use. I think that cost is certainly a barrier, especially in peacetime, but then one might see more open science, each thing that adds delay offers another quickening of the pace. The best thing might be a fundamental shift in Great Power relations, one that undercuts the funding, the impetus or the fear that drives defense procurement. You might have a disarmament movement be more successful and aggressive in curbing new weapons, the Washington Naval treaty did a lot to hamper the aircraft Carrier and rein back the Battleship from 1920 to 1935-ish. Even after, without the war those implements might have lost another decade of development and stagnated further. Have the first World War end differently such that a truer League of Peace occurs rather than a rearranging of chairs at the big table. A different Germany might pioneer nuclear science more fully, perhaps first in isotopes for industrial, medical or other peaceful uses, then power generation, perhaps the dangers of nuclear technology get revealed before a bomb is built yet after it is devised, sufficient to set forces upon it to contain it. The parallels exist but sadly all too often respond after the horse in question has bolted.