The UCM's origins began with the coming of theScourge. Though theCradle WorldsandEarthitself were conquered all but immediately, the various colony worlds of humanity remained somehow unharmed. It is unclear if the Scourge knew of their locations, or if they simply chose to focus on the biggest prizes that humanity could offer; in either case however, the colonies found themselves as the only safe havens for fleeing humans. Faced with the looming threat of the Scourge following the refugee ships, colonial governors chose to reconfigure theirfoldspacenodes, preventing further humans from reaching safety, but cutting themselves off from the invaders as well.
In the early stages of the aftermath, the United Colonies of Mankind was formed as a multi-planet federation to help coordinate the rebuilding effort. Under its leadership, the colonies were built up from frontier worlds into sprawling civilisations - a task made easier by exploiting the vast material wealth of the colonies to their full. The looming threat of a Scourge invasion also encouraged the nascent superstate to become strongly militaristic.