Nepleslia declared independence in YE 28 during the war between the Mishu and Yamatai. The post of independence can be found here:
https://stararmy.com/forums/viewtopic.php?t=1805
The independence of Nepleslia was a bloodless, peaceful affair. There are two theories behind why:
1. Yamatai was stretched too thin from the war with the SMX and let Nepleslia seceed because it couldn't otherwise prevent it or didn't want the responsibility to defend another vast section of space (making Nepleslia a buffer state)
2. Nepleslia and Yamatai shared strong relations, and the desire of Nepleslia to run its own affairs didn't raise any concerns with the empire.
3. Nepleslia secceeded to break away from the wrath of the SMX by declaring neutrality, and Yamatai, who really didn't want to put the effort into reclaiming the empire.
I tend to believe that it's mostly a combination of the first and third reasons, with a little of the second thrown in for public relations purposes.
After independence was declared, Nepleslia formed a governing body out of Emperor Jim Blackman and Robert Davis, leader of the Nepleslian army.
With the help of Emrys incorporated, Davis spearheaded several social programs to help improve the infrastructure of Nepleslia, resulting in the construction of state-of-the-art medical facilities, a strong transportation network and (slightly) better living conditions for the masses. (See:
https://stararmy.com/forums/viewtopic.php?t=2810)
The military also conducted a secret war against the gangs of Nepleslia (blacks and reds) and succeeded in weakening them.
Civil rights took both a step forward and a step backwards with the formation of a senate, which immediately took it upon itself to pass legislature that alienated the fledgling empire with both the citizens and the major weapons/ships corporations of the empire. It was a bill that created the entity known as DARTS, which monitored and taxed all weapons producers on the planets (for the purposes of eradicating crime syndicates once and for all), as well as tagged all weapons sales (an invasion of privacy). As expected, this backfired, and the Reds, who controlled the major weapons manufacturer for the empire, pulled out and reformed as a military opposition to the empire.
Popular opinion of the army plummeted and Emperor Blackman and Davis came to odds, their beliefs of how the nation should be run coming in direct opposition. (See: Freedom and Tyranny
https://stararmy.com/forums/viewtopic.php?t=3152. Although in the post I make Davis to look like the good guy, the matter is really a shade of gray (as Blackman could very well be expected to be furious as he saw his empire falling apart and his peaceful alliance ruined.)
To prevent collapse of the military strength of the empire, Davis formed NAMs as a government-operated company. Also, an alliance with the NDI was formed, which resulted in Nepeslia getting a pretty massive fleet upgrade (see:
https://stararmy.com/forums/viewtopic.php?t=3446).
Currently, there is massive work on helping to fix the Super Y problem with the population, as well as getting Nepleslia into space and expanding.
I hope this provides an adequate background to Nepleslia independence. If you have any questions, please ask.