An increasing number of local worlds from Year 12,800 onward, especially those affiliated with the party of Freethinkers, decided to overthrow the monarchy of the Star Imperium and create a republic. These rebellious planets pooled their Planetary Security Forces into a so-called Rebel Alliance in Year 12,834.
By Year 12,900, a small but growing number of members of the Judicial Forces, Intergalactic Navy, and Stormtroopers joined the Alliance in open rebellion.
By Year 13,050, they had won over most of the military and police forces of the Imperium in certain Stellar sectors and Subsectors. After that year, the Imperium relatively rapidly lost momentun.
In Year 13,100, Alfheim itself was invaded, and the ruling Imperial Sovereign Artei 20th and the Imperial Parliament (although by that time the Freethinkers had a parliamentary majority and all factions of the Monarchist Party were in serious decline in Parliament) were forced to cede power to a new intergalactic republic, the United Planets.
Political power was ceded to the United Planets Assembly, a Council of Twelve randomly selected from the ranks of the United Planets Assembly rather than the Imperial Parliament, and a Prime Minister and Council of Ministers elected by the Assembly.