After Earth's natural resources were used up generations ago, mankind abandoned the planet (which now exists in myth and legend as "Earth-That-Was"), and moved out into the stars. But, mankind held onto the tenets of culture and civilization in the exodus, carrying with it those things both material and spiritual that make us human.

Relatively little is known of the early history of the Alliance (such as the exact date of the exodus from Earth, and the reasons why China and the USA allied themselves with each other). The Alliance flag, seen in the original pilot episode, is a blending of the United States and People's Republic of China flags (two versions of this flag, one with 13 stripes and one with five stripes, appear in the series).

At the Battle of Serenity Valley (the last major battle in the civil war, based on the Battle of Gettysburg), both the Alliance and the Browncoats suffered heavy casualties, particularly amongst officers. Survivors from both sides were stranded on the battlefield until the Alliance was able to send a fleet of medical transports to rescue them.

The day that the civil war ended is known as "Unification Day" (often abbreviated to "U-Day") and is celebrated as a holiday throughout the Alliance. Alliance military officers like to drink a toast to Unification Day and enthusiastically encourage others to join in the festivities.