In that context [clear Soviet superiority of conventional forces in Europe], the fact that it had been the United States which had been the first to successfully develop nuclear weapons (and use them in anger, a decision which remains hotly debated to this day) must have seemed like an act of divine providence, as it enabled the western allies to retain a form of military parity with the USSR (and thus deterrence) while still demobilizing. US airbases in Europe put much of the Soviet Union in range of American bombers which could carry nuclear weapons, which served to “balance” the conventional disparity. It’s important to keep in mind also that nuclear weapons emerged in the context where “strategic” urban bombing had been extensively normalized during the Second World War; the idea that the next major war would include the destruction of cities from the air wasn’t quite as shocking to them as it was to us – indeed, it was assumed. Consequently, planners in the US military went about planning how they would use nuclear weapons on the battlefield (and beyond it) should a war with a non-nuclear Soviet Union occur.

At the same time, US strategists (particularly associated with the RAND Corporation) were beginning to puzzle out the long term strategic implications of nuclear weapons. In 1946, Bernard Brodie published The Absolute Weapon which set out the basic outlines of deterrence theory; he did this, to be clear, three years before the USSR successfully tested its first nuclear weapon in 1949 (far earlier than anyone expected because the USSR had spies in the Manhattan Project). Brodie is thus predicting what the strategic situation will be like when the USSR developed nuclear weapons; his predictions proved startlingly accurate, in the event.

Brodie’s argument proceeds as a series of propositions (paraphrased):

1. The power of a nuclear bomb is such that any city can be destroyed by less than ten bombs.
2. No adequate defense against the bomb exists and the possibilities of such are very unlikely.
3. Nuclear weapons will motivate the development of newer, longer range and harder to stop delivery systems.
4. Superiority in the air is not going to be enough to stop sufficient nuclear weapons getting through.
5. Superiority in nuclear arms also cannot guarantee meaningful strategic superiority. It does not matter that you had more bombs if all of your cities are rubble.
6. Within five to ten years (of 1946), other powers will have nuclear weapons. [This happened in just three years.]

All of which, in the following years were shown to be true. Consequently, Brodie notes that while nuclear weapons are “the apotheosis of aggressive instruments”, any attacker who used them would fear retaliation with their enemy’s nuclear weapons which would in turn also be so destructive such that “no victory, even if guaranteed in advance – which it never is – would be worth the price”. Crucially, it is not the fact of retaliation, but the fear of it, which matters and “the threat of retaliation does not have to be 100 per cent certain; it is sufficient if there is a good chance of it, or if there is a belief that there is a good chance of it. The prediction is more important than the fact.” [emphasis mine]

This does not “make war impossible” by any means, but rather turns strategy towards focusing on making sure that nuclear weapons are not used, by making it clear to any potential aggressor that nuclear weapons would be used against them. And that leads to Brodie’s final, key conclusion:

Thus, the first and most vital step in any American security program for the age of atomic bombs is to take measures to guarantee to ourselves in case of attack the possibility of retaliation in kind. The writer in making that statement is not for the moment concerned about who will win the next war in which atomic bombs are used. Thus far the chief purpose of our military establishment has been to win wars. From now on its chief purpose must be to avert them. It can have almost no other useful purpose.

To sum that up, because both the United States and its key enemies will have nuclear weapons and because their destructive power is effectively absolute (so high as to make any “victory” meaningless) and because there is no effective defense against such weapons, consequently the only rational response is to avoid the use of nuclear weapons and the only way to do that is to be able to credibly threaten to retaliate with nuclear weapons in the event of war (since if you cannot so retaliate, your opponent could use their nuclear weapons without fear).

That thinking actually took a while to take hold in actual American policy and instead during the 1940s and 1950s, the United States focused resources on bomber fleets with the assumption that they would match Soviet superiority in conventional arms in Europe with American nuclear superiority, striking military and industrial targets (“precision attacks with an area weapon”, a notion that is as preposterous as it feels) to immediately cripple the USSR in the event of war, or else aim to “win” a “limited” nuclear exchange.

Bret Devereaux, “Collections: Nuclear Deterrence 101”, A Collection of Unmitigated Pedantry, 2022-03-11.