When humans jettisoned exploratory ships into space, first contact seemed imminent. We were barely leaving an era of nationalistic isolationism, yet the twinkle of hope that we are not alone in the universe burned brightly in the hearts of humans around the world.
Almost every planet explored was brimming with life, but there were no civilizations, not even the traces of one long gone. Several planets were colonized and developed according to their resources and needs. Many were dependent on supplies from Earth. Unbeknownst to the colonies, those supplies usually came from other colonies. The ballooning population of Earth became manageable as mountains of people were sent to colonize the new planets.
When World War Four broke out, the colonies saw the most fighting. Factions vied for territory to the detriment of its citizens. While at first swept up in a historic nationalism, it became apparent to colonists that their cultures had diverged somewhat from their Earth-born camarades. They looked down on the colonists and did little to help despite the strain of supply shipments due to their war. Only the faction of Independents continued their regular supply shipments, with the Alliance and Federation prioritizing their war efforts.
When the Independents were pulled into the war, they gave the colonies details on the colony resources and how Earth had been distributing them. The colonies began to work with one another and the worst of their suffering came to an end. Enraged by their treatment at the hands of Earthers, they declared independence and rebelled, ambushing Earth troops in the colonies. Nobody fretted about the faction, only that they were Earth troops.
Alliance and Federation troops fought to hold their positions, at least the ones which had not been overrun, but their efforts were futile. With such a grip in the colonies lost, all factions were forced to retreat to Earth. The war did not last much longer, and Earth forged a peace while the colonists put together what was left of their homes.