125 years ago: Earth (at the time in a 1900-1920 equivalent industrial age) was bombarded by a meteor shower. The destruction, and resulting effect on the climate was devastating, and nearly "broke" civilization back to a dark age.
120 years ago: People began manifesting Psychic abilities.
100 years ago: Alien ships attacked. Earth was unprepared, and again the landscape battered. Then suddenly the ships came crashing down to Earth. The alien attack lasted one day, and there has been no sign since.
95 years ago: Most governments have collapsed. The fractured military looks to corporations for guidance. They surely would know best how rebuild with efficiency.
90 years ago: Each city becomes ruled by a corporation or conglomerate as a monarchy. Citizens are employees or exiled.
The past 90 years:
The isolationist maneuver was "sold" to the people by pledges of humanitarian efforts toward those who had decided to not conform to this new way of life. The civic leaders of settlements - built upon ruined cities - received vitamin rich rations to supplement their citizen's diets. They also would at times receive discarded self-defense weaponry from the rapidly upgrading city watch. This went on for a while, until the inevitable conflict came - brought by wealth surrounded by destitution.
Rebellion came in the form of guerilla attacks.. supply caravan ambushes.
There were enough reportable incidents to sanction entire settlements. One by one these communities were cut off from civilization. They were left to fend for themselves, and defend what resources they had left from desperate plunderers and murderous gangs liberated by the decline of civilization.
Okorn Spring is the only settlement still receiving rations and medicine from the nearest city kingdom of New Anodos (Anotheurge Multi-Corps). Most assume this is due to Okorn Spring's leader, Bergen Lachlan, being a brother to Kandur... the "Farm Lord" of New Anodos.
Nature has begun to recover, and the denizens of the world outside the walls have adapted in the generations since. Most who still survive no longer have need of assistance. But there are those that band together for a common good. While certainly... not everyone has the same motivations, there is a focus to these well-armed and fairly well-intentioned groups based on a hope. There must some way or somewhere they can live their last days believing their kin will have a better life.