The Commonwealth of Independent States, commonly known either as simply the Commonwealth, or abbreviated as the CIS, was a governing body formed in 55 BBY, composed of systems and corporations that broke away from the Galactic Republic as a result of the Republic's perceived inability to defend such systems if the events of the Mandalorian Civil War spread into non-Mandalorian sectors. The Commonwealth's early financial and political leadership was derived mainly from the InterGalactic Banking Clan, the first corporation officially secede from the Republic. The InterGalactic Banking Clan's financial power and the persuasive charm of the company's chairman, San Hill, caused many more systems and corporations to follow in the Clan's footsteps, isolating themselves from the Republic. As the number of disenfranchised and secessionist systems grew, San Hill united them into a political entity in order to challenge the Republic, which the newly-formed Commonwealth viewed as stagnant, corrupt and hesitant.
In 52 BBY it was discovered that the Commonwealth was building up a powerful military force, composed primarily of clone soldiers. Numbering in the quadrillions, the clone forces of the Commonwealth were seen to be large enough and powerful enough to be viewed as a significant threat and sign of aggression towards the Republic, and the discovery of this army was the prime reason that the Galactic Clone Wars began.
The Commonwealth was dismantled in 19 BBY, after the Republic won the drawn out Clone Wars.