system
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human
[LOTR] Who actually protects the Shire?

Like I mean, there does not seem to be any army, fortifications or police force present in the Shire.  I mean, its pretty evident that there are a lot of threats in Middle Earth, from Goblins, Orcs and Trolls to bandits, and human armies.  I mean, the Nazgul, who pretty clearly had ill intentions, literally just strolled in.  

Despite all this, the Shire seems incredibly peaceful.  How does this work?
gpt
You could say the Shire has three layers of defences that have allowed it to remain peaceful despite all of the other problems that Middle-earth experieces. 

The first and foremost layer of defence is the Shire's location, as well as the nature of its inhabitants. The Shire is small, and far from any of the major conflicts of Middle-earth. Along with that, Hobbits are pretty insular, so they usually don't leave the Shire, nor does anyone (apart from the occasional Elves or Dwarves) enter the Shire. As a result, most people don't even actually know where the Shire is. When Sauron and the Nazgul were told that the One Ring was in the Shire, they didn't even know where to look. They searched through Rohan and even the Vales of Anduin, and the only reason they ended up finding the Shire was because they came across Grima Wormtongue, who gave away the location. So if Sauron and the Nazgul don't even know where the Shire is, chances are most other evil folk don't know either. Can't invade a place if you don't know about it. 

The second, and probably the most practical layer of defence is the Dunedain (the Rangers of the North). They've been protecting the Shire (as well as places like Bree) ever since the fall of Arnor, and have likely turned away most of the threats that the Shire may have faced. Considering the Shire's prosperity, they're usually very effective, but not infallible. Orcs and Wolves have managed to make it into the Shire at certain points, and the Nazgul would break through the Rangers' defences at Sarn Ford in the prelude to the War of the Ring. Saruman's success in eventually taking the Shire is largely due to many of the Rangers being tied up by other issues. 

The third layer of defence is the Bounders, the Shire's "police force". They wouldn't be able to stand up to a proper invasion, but they're brave, and certainly aren't pushovers. About 250 years before the War of the Ring, a warband of Orcs were defeated by the Hobbits led by Bandobras Took in the Battle of Greenfields. The Bounders ended up being pretty ineffective when Saruman took control, but a large part of that was due to internal betrayal by Lotho Sackville-Baggins.