1. The beginning
It had been known for thousands of years that an Earth-like planet existed within Andromeda cluster. However, due to limitation of spacecraft propulsion tech, it wasn't until early year 8xxx that an actual visit was possible without cryogenics.
Even when it became posssible to reach the cluster in half a year, an actual colonization wasn't pushed by the Ark due to astronomical cost involved.
It took a strong leader to push the distant colonization and it would take Gawain Klisis to push the agenda.
Having united the Bau and the O'ren under one banner, Gawain Klisis published a plan to colonize Andromda cluster.
2. Liberty captains
Before full scale colonization would begin, the cluster needed to be thoroughly explored. Instead of using the navy to explore which would be very costy, Gawain created liberty captian program which was about letting civilians become captains of a single cruiser and let them explore on their own. This method took hundreds of years and, unfortunately, Gawain passed away before the actual colonization would begin.
However, small scale colonization did begin and the Earth-like planet was named "Heaven of Order" for the time being before being given an official name of "New Earth" much later.
3. Full colonization and then rebellion
Hundreds of years later, it was President Mirren who initiated full colonizaton with Cecil Klisis as his top adviser.
However, there was an issue with taxes.
In order to entice immigrants, Gawain offered complete tax exemption for all applicants. Millions of initial wave of immigrants abandoned their homes and re-started their lives anew in Andromeda cluster.
Due to rising cost, Mirren had to impose taxes on the immigrants who were, at this point, no longer immigrants but were natives to Andromeda cluster. The first tax was 3% and then 6%. It kept on rising to nearly 20% later on when the relationship between the Ark and Andromeda residents was at a boiling point.
Finally, on 9077, a rebellion was declared. And the rebellion was successful on 9098. Thus, Andromeda republic was created by Acshell the liberator. The Ark sort of acknowledged their existence on 9124.
The full, 74 pages, story behind the rebellion is covered in [Legends arc] [Andromeda rebellion] .
4. Political structure
Acshell the liberator wished to form a republic and announced that he'd step down once a formal election was held. The public was against the idea and wanted Acshell to form a dictatorship due to their complete trust in him.
However, before an election was held, he vanished without a trace, leaving everything in limbo for a while.
A Bau exile, Richard Bau, took control and formed a union. Since he was a core member under Acshell, the public accepted him as a rightful leader.
After Richard Bau formed a government, Andromeda union was controlled by two chambers of council (Low & High). Richard Bau himself partook in virtually no politics but remained to be the surpreme leader until his death.
5. National emblem
Andromeda union's emblem was a group of upward fists, one of it holding "AA". The "AA" meant "Andromeda union by Acshell the liberator".
The upward fists were symbols of middle class fighting against elites.
6. Low & High council
Low council was simply referred as council and its functions were identical to United Sol's house chmaber.
Low council had 200 members. A major difference compared to United Sol's house was lack of clan intervention.
As for high council, its functions were significantly different compared to United Sol's senate. High council was consisted of only 12 members and they were in charge of Andromeda imperial fleet and even included spaceship related R&D.
7. The Grand Agenda
Hatred toward United Sol from Andromedians were common and profound. The way they were treated, the way they were looked down, and the way they were overtaxed, it all added up to their hatred.
Juron took advantage of this hatred and started what historians call "The Grand Agenda". Until Juron was killed by Rear Admiral Suu in order to revenge Grand Admiral Kain, he had called a total of four invasions on United Sol.
On his forth, and final, invasion, Juron was very close to completely conquering United Sol. He had killed Grand Admiral Kain and he had a secret agreement with Emuel for surrender. However, led by Rear Admiral Suu, a small but highly determined fleet stroke Juron's core fleet with element of complete surprise. Suu's own commandship rammed Juron's commandship, killing Juron on the spot. Suu also perished as well, ending Juron's grand agenda once for all.
These four invasions by Juron had profoundly negative effects on already ailing United Sol which they would never recover.
Andromeda union would never invade United Sol again and some decades later, Cecil Klisis finished it off.