system
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human
[Star Wars] How could star systems slip through Tarkin's fingers?

How could a star system openly rebel or even moderately help the Rebellion without the Empire finding out and sending a fleet to ravage the planet? The entire planet's population would need to keep a secret, which isn't reasonable. Is the Rebellion strong enough to hold territory against the Empire to protect key systems? The only way I can see this happening is if multiple systems simultaneously rebel so that the Imperial fleet is spread too thin but simultaneous Rebellion is unlikely.
gpt
The Empire's main concentration of power was in the Core Regions and Inner Rim, which is most of the territory controlled by the Republic during the Clone Wars, and was the historical center of power for the galaxy for millennia. Countries in the Mid and Outer Rim were more likely to have sided with the CIS during the Clone Wars, and Imperial influence was both weaker and more recent. The Empire spent quite a few years chasing down the last of the Droid Armies, solidifying their hold in the Core Regions, and building up the Imperial Bureaucracy. 

For planets in the Outer Rim, a Rebellion had a much greater chance of succeeding. Some planets still had little to no Imperial authority (such as Tatooine) or only a small presence. Any rebellion would take much longer to respond to, and would likely not be able to deploy the same overwhelming force they could in the Core regions. 

While no planets were in open rebellion at this time, they soon would be. The aftermath of the Battle of Yavin saw widespread galactic protest at the destruction of Alderaan, and the Rebel victory proved to many planets on the fence that the Rebels could pose a serious threat to the Empire. 

Imperial forces were quite honestly, stretched very thin. In Legends, we know that the Empire had 25,000 Star Destroyers at it's peak, with many more smaller support ships. The galaxy had about 1000 sectors, each with dozens of star systems and hundreds of planets. That's 25 destroyers per sector, not counting the other destroyers that might be protecting key planets such as Coruscant or on deployment elsewhere, and-for the sake of argument-one destroyer per star system. 

If a planet rebelled, it could be some time before the Empire could respond to the threat. In many cases, they would likely have to deploy a destroyer that was on duty eleswhere to respond to that threat, which would of course, leave another planet unguarded. If the Empire tightens their grip, planets that would otherwise not act out might be drawn to Rebellion. Instead of sending just one destroyer, they now send two, and make an example of this system. This in turn, drives more systems to rebel, and requires the Empire to exert greater and greater pressure, but also leave more and more systems unguarded. The Empire can keep putting out fires and shuffling around ships, and keep rebellion contained, but once it reaches a certain point, the Empire can't respond to every threat without weakening their grip. Eventually, the Empire has to start making choices. Do you let the developed, industrial world that is refusing to pay their taxes get away with it? Or do you go all the way out to the Outer Rim to deal with the tiny agricultural planet that is openly declaring their allegiance to the Rebellion. Logic say to deal with the former, and let that tiny system slip away. And it begins to snowball away.