The Rusting Age followed the collapse of civilization.
Like the era of collapse before it, the Rusting Age did not happen everywhere all at once, and so they have no definitive start or end date. Generally speaking, though, the period generally aligns with the twenty-second century of the old Gregorian calendar.
By the time that civilization collapsed, the planet's natural soil fertility had already dramatically declined, removed by ten thousand years of agricultural production. Crops still grew only thanks to massive inputs made possible by fossil fuels, and as those became unavailable agriculture became untenable. Climate change only worsened an already dire situation.
Nonetheless, people persisted in trying to survive as they always had. Most starved to death. In a few pockets, though, they found soil that remained fertile and managed to grow crops. Wherever people could find such a pocket, a community would form. The very agricultural practices that these conditions permitted led to their downfall, however, as they would degrade the soil there and, within a generation or two, leave it as barren as the rest of the land around it. The Rusting Age saw many such agricultural communities form, take root, and then die, consuming the last areas where that way of life could still work. After that, centuries would have to pass before agriculture would become viable again.
In the ruins of the old cities, local strongmen emerged from the chaos. They needed to unite several disparate communities that often had a history of violent conflict. They needed to provide food for their communities, which became an increasingly daunting proposition. Very often they employed violence to achieve these goals. They often invoked the titles and symbols of authority from the former days of their civilized past, styling themselves as Presidents, Prime Ministers, Governors, or Mayors, even though the reality of their rule more closely resembled that of a feudal lord.
They often relied on slave labor to mine the ruins of civilization for scrap metal. Without fossil fuel they could not burn fires hot enough to smelt most iron alloys, including steel. They could heat some metals enough to reshape them, but they mostly had to rely on unalloyed iron. Over time, they recovered lower and lower quality metal from the ruins, both because they had already extracted the best metal and because as more time passed more of the metal became exposed to the elements and rusted away. Rust takes the form of iron oxide, the same as iron ore, and so they could smelt it for more metal, but as a practical matter they lost a great deal of it in each iteration of this process. Likewise, as old tools, weapons, and armor would rust, they would rework them, but they would inevitably lose much of the material through each iteration. This led to the eventual name for the period — the Rusting Age, meaning the time during which the metals left behind by civilization rusted away.
As the metal sources ran out, the warlords found it increasingly difficult to equip their personal armies. The decades or even generations of hatred and resentment that their cruelty inspired led to an era of revolutions, rebellions, and insurgencies. By the middle of the twenty-third century, the warlords of the Rusting Age had become little more than bitter memories and the villains in oral histories.
Without fossil fuels to fuel vehicles, the army of a neo-feudal warlord could only march as far as they could provision themselves — both for their mission, and their trip home. They could rarely go more than 150 miles before they would have to turn back. Anywhere that lay 200 miles or more from the nearest city ruin, then, could exist in a world that knew nothing of the slavery, desperation, cruelty, and violence in the neo-feudal kingdoms. In those areas, people began living by hunting, gathering, and gardening much earlier than people elsewhere.