history

You never hear about murder hornets in the usa bc the department of agriculture hunted down and killed all of them.

The Carrington Event occurred before usage of electricity was so ubiquitous. If a coronal mass ejection of that size or larger were to happen now, large sections of the planet could be without power for weeks. Given how dependent we are on electricity, that could lead to a societal collapse.

Look up the work of Norman Borlaug. In the 1940s through the 1960s he developed and spread high-yield agricultural techniques and crop varieties. He travelled to some of the poorest most underdeveloped countries to share his knowledge and increase the yields of farmers there.

In the early 1900s many scientists and other figures had been convinced we were facing a looming Malthusian crisis in which millions would simply starve. Norman Borlaug effectively averted that crisis for countless millions of people worldwide.

Castro wasn't assassinated, Bay of Pigs failed, years of terrorism and biological warfare failed to defeat Cuba's revolution. Likely not world spanning impacts but still pretty cool.

There was that one time the Soviet early warning system malfunctioned but some brave comrades chose not to launch.

Or that one time that a plane crash almost nuked North Carolina.

Essentially if the many worlds hypothesis is correct, the multiverse is littered with the charred remnants of mid-century Earths where luck didn't go our way.

The final Tsar's grandfather died in a train crash because he was blind drunk and made the engineer go faster and faster until it derailed. Probably accelerated the fall of the Russian empire a little bit.

The preparedness paradox is something we run into a lot these days, and is related to your question. When you avert a crisis people will inevitably point and say “See, nothing happened! You wasted all that time, money, and effort for nothing!” Our collective Covid response was an example, the vaccines and lockdowns were demonstrably effective in reducing deaths and yet millions who were merely personally inconvenienced insist it was all a waste of time.

As far as big “smoking gun” examples of narrowly averted disasters it will probably be hard to find many good examples, because as you said historians tend not to write about things that didn’t happen.

That being said, some of them absolutely love to speculate on alternate scenarios, and there is no lack of examples of crises or events that were not averted, but we can point directly at actions people took that prevented the situation from being even worse than it was.

Not a specific event or policy, more a broad institution of the ancient world (it’s codified in the Bible), but I’d argue that the practice of the debt jubilee averted crises. Both in that the discharging of debts acted as a release valve for the underclasses, but more importantly that they prevented any one debt collector or issuer of debt from amassing too much economic, and ergo political, power. So there was likely a lot of strife, power grabs, violence, etc., that was avoided because of those regular erasure of debts.

The Acadian Expulsion. Brits doing ethnic cleansing of francophones on Canada's east coast.