Earth-that-was could no longer sustain our numbers, we were so many. We found a new solar system - dozens of planets and hundreds of moons. Each one terraformed -- a process taking decades to support human life to be new Earths. The central planets formed the Alliance. Ruled by an interplanetary parliament, the Alliance was a beacon of civilization. The savage outer planets were not so enlightened and refused Alliance control. The war was devastating, but the Alliance's victory over the Independents ensured a safer universe. And now, everyone can enjoy the comfort and enlightenment of true civilization.
--- River Tam's teacher, Serenity
Long ago, on the Earth-that-was, there were a few people who saw the future of the planet as it truly was: doomed. They endeavored through doubt and denial, sending automated terraforming machinery to prospective planets. Years later, in the hopes that their actions had been fruitful, hundreds of multi-generational ships traveled to these planets in the hope of continued life.
Terraforming was only the first stage. The most dense cluster of planets received the largest support in terms of cargo and personnel. These planets became the central planets and formed themselves into an Alliance. Outlying settlements did not receive the same level of support with the result of thse border planets being fairly dangerous and forbidding environments.
Hundreds of years passed as humanity struggled to create their niche in the new worlds. Once this was established, many started to look at their neighbors in the hopes of forming permanent alliances. The central planets stood to gain the most in terms of trade and manufacturing. In many ways, the propsed alliance was only a formalization of the pact between several large families with almost unlimited control and influence.
Feeling the proposal would serve these families at the expense of the outlying planets, the Border planets chose to refuse to join the Alliance. The resolve continued, despite increasing economic and medical embargos, designed to cripple the settlements. Facing vastly larger numbers, superior technology and a unified front, the Border planets resisted what they saw as self-serving propaganda. In the end, after several negotiations led to deaths, the Border planets, calling themselves Browcoats, declared they would never become part of the Alliance. Tensions and violence escalated. War was declared.
Despite easy predictions of defeat, the Browncoats held themselves against the Alliance far longer than any had dared to hope. But, it was in 251, in the Battle of Serenity Valley, that the war was ended. Almost a quarter of a million people died in the valley after four months of continued fire. After a particularly crushing defeat, the Browncoats offered surrender. They had lost.