Up until 2298, the Commonwealth of Man was the only nation of humanity since the earliest days of human colonization. While the population was, for the most part, supportive, there were certain elements that felt stifled and oppressed under the Commonwealth’s rule. As these feelings built up, dissatisfied citizens began to organize, acquire resources, and band together.
Over the course of years, simple protest evolved into active resistance. The government came down upon those who disagreed with them with steadily harsher punishments, which in turn further fed the growing dissent. In 2295, a mass execution of anti-government saboteurs tipped things over the edge, igniting a full-scale rebellion. Using contacts within the defense industry, the rebels acquired an army and navy from independent merchants, and fought a bloody civil war with the Commonwealth military, who had been initially confident that they would quickly and effeciently put down the uprising; a three year campaign and millions of casualties taught them otherwise.
Eventually, the Commonwealth capitulated, allowing the rebels to maintain their independence so long as they left Commonwealth space. This resulted in a mass exodus from the Orion Arm, with trillions of people leaving to start a new life. They traveled down to the Perseus Arm, a previously unexplored portion of the galaxy, and began terraforming planets and establishing a new government: the Independent States of Perseus. They grouped into separate polities, uniting under a ruling council for general direction, while making specific laws for themselves that only applied to their own citizens.
The ISP now focuses on expanding, establishing colonies in the far-flung reaches of the galaxy. Whereas most other imperialistic nations focus on acquiring known territory, the ISP do their best to distance themselves from outside influences, searching for habitable worlds in unknown space to avoid political squabbles with other factions.