One answer that is possible though not a popular idea is 'The UN.' More broadly it's the international governance that the UN is a part of. The UN's goal is/was largely preventing a nuclear WWIII, so it's at least worth considering.
It's easy to discount the UN. It's ineffective, self righteous without introspection, a paper tiger, a cynical dealmaking forum, undemocratic, etc. etc..
But… There are two influential UN agendas that have been to some extent, effective: Non Aggression and Non Proliferation.
Nonproliferation was a limited success. Only Pakistan, India, Israel & North Korea went nuclear despite it. Those are worrying, but do not amount to inevitable nuclear war.
If nuclear weapons were sold in the arms markets like ICBMs, fighter jets, cutting edge anti missile/aircraft defense systems, etc. then the list of nuclear powers would be much bigger than it is today. Sadaam's Iraq, Iran, Syria, Egypt & Saudi Arabia would have them (they have everything else). That's just one neighborhood. Non-State nuclear powers would be far more likely. 911 might have been a nuclear attack.
Non aggression is the second principle/agenda. Boiled down it means 'No Wars of Conquest'. If you conquer territory that is "rightfully" yours because ancestors or whatnot, no country will recognize your territorial gains ever.
Non Aggression has also been a limited success. Israel in 1967 is the high profile counter example. Russia in Ukraine is the bigger and more recent one. There have been other violations, but compared to the pre-UN period it is a different paradigm.
Instead, we have a paradigm of "intervention" and occupation instead of conquest or colonialism. It's not ideal but it does change the whole incentive-dynamic. No country's political discourse is dominated by the idea of expanding. That is uniquely true of the last 3 generations and no other period.
In that sense, the UN Non Aggression principle has successfully taken the spoils out of war and prevented the cyclical warfare which created the two Great Wars.
Ukraine is the counterexample and may be the end of the era. Who knows.
TLDR: The UN lowered incidence of wars between major powers via the "non aggression rule" and the proliferation of nuclear weapons to minor powers and non state actors via non proliferation initiatives. These two have been working together to keep the chances of nuclear war down, but it's a shaky system.
It's easy to discount the UN. It's ineffective, self righteous without introspection, a paper tiger, a cynical dealmaking forum, undemocratic, etc. etc..
But… There are two influential UN agendas that have been to some extent, effective: Non Aggression and Non Proliferation.
Nonproliferation was a limited success. Only Pakistan, India, Israel & North Korea went nuclear despite it. Those are worrying, but do not amount to inevitable nuclear war.
If nuclear weapons were sold in the arms markets like ICBMs, fighter jets, cutting edge anti missile/aircraft defense systems, etc. then the list of nuclear powers would be much bigger than it is today. Sadaam's Iraq, Iran, Syria, Egypt & Saudi Arabia would have them (they have everything else). That's just one neighborhood. Non-State nuclear powers would be far more likely. 911 might have been a nuclear attack.
Non aggression is the second principle/agenda. Boiled down it means 'No Wars of Conquest'. If you conquer territory that is "rightfully" yours because ancestors or whatnot, no country will recognize your territorial gains ever.
Non Aggression has also been a limited success. Israel in 1967 is the high profile counter example. Russia in Ukraine is the bigger and more recent one. There have been other violations, but compared to the pre-UN period it is a different paradigm.
Instead, we have a paradigm of "intervention" and occupation instead of conquest or colonialism. It's not ideal but it does change the whole incentive-dynamic. No country's political discourse is dominated by the idea of expanding. That is uniquely true of the last 3 generations and no other period.
In that sense, the UN Non Aggression principle has successfully taken the spoils out of war and prevented the cyclical warfare which created the two Great Wars.
Ukraine is the counterexample and may be the end of the era. Who knows.
TLDR: The UN lowered incidence of wars between major powers via the "non aggression rule" and the proliferation of nuclear weapons to minor powers and non state actors via non proliferation initiatives. These two have been working together to keep the chances of nuclear war down, but it's a shaky system.