Spoilers
Do not read this unless you've read all the chapters and only if you want to make sure your conclusions are those I intended (it doesn't mean your version isn't the right one, just that it wasn't the one I had in mind for this Fanfiction).
Things possibly intended in the show:
The story takes place in Virginia.
Mount Weather's underground facilities were used as intended for the Continuity of Operations Plan.
The Grounders are the remnants of the National Guard and still guard the facilities from any outsiders - they ran out of bullets long ago.
Things proper to the Fanfiction:
Darian is the descendent of the President of the United States, a fervently religious leader. (His "crest" is actually the seal of the president. He is always looking at this passage of the bible, because he tries to understand the acronym "POTUS", thinking it was a word. (Trivia: Potus in Latin gave the word Potion in English.))
The Others (part of the Grounders) pretty much decided they didn't have much more to give to their country since it doesn't really exist anymore.
The NN scenario (part of the Fanfiction):
The "No Nukes" scenario is that there was no nuclear holocaust.
The rise of the sea level lead to less habitable land while the population kept growing. Overpopulation put a strain on international relations. The United States Capitol is underwater.
The approach of a "global meteorological cataclysm" became more and more obvious, be it a worldwide drought or a new ice-age. The G8, under the leadership of the USA, began a fail-safe program known as "The Ark", obviously inspired by biblical events.
In an attempt to rekindle Earth's ecosystem which is slowly dying, the scientific community miraculously achieved a system to "overgrow" nature, with the vast possibility of doing things wrong (thus two-headed deer, no radiation poisoning, spread of forests and glowing butterflies, because no, there is no effing Cherenkov effect on the wings of a bug).
The Ark was loaded with a large population from all over the world (because inbreeding is a B-word) with a simple rule: one children only, thus reducing the population to a mere 800, two centuries later.