Our ancestors tell us that they ended the world, as they knew it, by accident and malice. A 'focused interest group' shot down a transport plane that belonged to the government that they were agitating against. The plane was carrying multiple cargos, including a government minister and multiple city dosages of experimental population control psychotropics.
Yes, I know, heavier than air flying machines and mass medication. You couldn't make these things up.
The psychotropics were supposed to be delivered via the processed water system as part of a cocktail designed to control the active dosage to each recipient. Being volatilised into the naive water cycle would not have been a recommended delivery method. Particularly not without the dosage inhibitors it was supposed to be bundled with. You can imagine the types of effects that had on the non-target species in the area: rain in the region was lethal for certain reptiles; drinking contaminated water was fatal to some creatures; predators tended to be more risk adverse; and ruminants were noted to be more aggressive.
Another fact you couldn't make up: the local water filtration systems although designed to permit these things to potentially be added to the water, weren't designed to take them out of the water.
The towns nearest the exploded plane were too small to have population mood control in place, and so didn't have dosage inhibitors in their water to limit the uptake of the drug when it started coming through the taps. The government that lost the plane didn't warn anyone that there might be a problem; we could be generous and say that they thought the fireball had destroyed the pharmaceutical cargo. Surviving records show extensive eulogies for the murdered minister.
Deer and feral goats started attacking people in the woods. Farm ruminants and their farmers became more closely bonded, but there were some districts where you didn't want to go if you weren't a local. Horses started avoiding cows, sheep and goats. In the local towns, public forums started filling with conspiracy theories.
The water cycle, being the water cycle carried on. Unmetabolized psychotropic continued to travel with the water where ever it went. This was in back blocks country, places whose minutiae could be ignored in the process of running the country, until a little town seized control of the nearby power station and shut it down.
Because, they said, the electromagnetic generators in the plant were sending instructions to the people trying to steal their children.
The resulting power outage prevented the addition of dosage inhibitors into the water in four major cities for three days. Actions by paranoid groups in those cities began on the third day, and a favourite target was the water purification and dosing system. Untreated, unmedicated water continued to flow in those cities. More small towns started getting dosed with unrestricted psychotropics. 'Activists' from the affected cities travelled to other cities to sabotage their infrastructure. The problems spread.
Vast areas of countryside became unsafe if you didn't 'belong' there.
Any forest with a deer population or mountains with goats became problematic.
The government either refused to admit to the cause of the problem, didn't know what had caused the problem, or were already affected, so didn't care.
Water being no respecter of persons, and subject only to the laws of physics, continued to evaporate, drift around as cloud, then fall to the ground wherever it found itself.
People across the world became increasingly paranoid, and then someone launched a pre-emptive retaliatory attack on an old enemy. Their world continued to function long enough for the launcher to be court-martialled, and their excuse was, "They were looking at us funny. They were up to something."
That is one of the last coherent documents on record for that hemisphere in that period. Within days of that record further weapons of mass destruction were deployed, and our ancestors reduced themselves to incoherently screaming at each other for another decade. We can only speculate what other drugs they may have volatilised as they 'neutralized' enemy targets.
Truth in language seems to have been an early victim of that war.
The end of the war was an exhaustion of resources. All spent, it took our ancestors another four decades to craft the Declaration of Peace that graces every Wall of Remembrance. The Declaration is the aspiration on which we build our present and our future. The past was full of many marvels, but so is the present, and I personally understand how gravity servos work, but am flummoxed by the idea of flight using airflow over asymmetrical wing profiles.