Humanity's home lies barren and depleted, racked by centuries of war that ended in a nuclear holocaust.
This catastrophe stemmed from a resource war between two global powers.
The Mugong Compact comprised of Central - Eastern Eurasia, and Africa. Fighting against the Western Mega-Corporation comprised of Australia, the Americas, and what's left of Western Europe. These devastating wars pushed human civilization underground and forced them to live like aliens on their home-world.
Upon discovering fusion technology, in isolated underground labs, human civilization saw a veritable boom. This explosion in population and infrastructure was caused, and was accelerated by a New Space Race. Humanity looked for resources outside of their barren home-world, leading to the colonization of the moons of Earth, Mars, and parts of the Asteroid Belt.
Prior to the Colonization of Mars, a series of space-mining rebellions led to the secession of an off-world faction currently known as "The Explorers". This rag tag bunch of pilots, traders, and miners, led by the pragmatic Damon Barnes have maintained autonomy in the face of Earthling aggression, proving themselves to be tenacious fighters.
Some say the colonization of Mars was predicated by this secession, others say it was the logical next step for humanity in their quest for stellar hegemony.
I take a lot of influence from Star Trek, the "Alien vs Predator" universe, Halo, Titanfall (the Apex: Predators universe), Cowboy Bebop, Akira, Cyberpunk: 2077, and Star Wars.