So the Rangers protecting the Shire was just a part of an overall Middle-Earth protection system?
I remember it that way, at least for the parts that couldn't really protect themselves too well. But sort of undercover, didn't someone liken them to the CIA?
Or the Jesuits. ;^)
That is the way I think of them. Just as Morgoth in the olden days, Sauron was in the habit of continually sending out spies to find out what was happening in the land. The Rangers were there to intercept, assassinate, mislead, and or divert them as seemed best. Ever vigilant they were in the protection of the free lands... "Eternal vigilance is the price of Liberty"--(wasn't it Barry Goldwater who said that?)
I don't think it was just the Shire they were protecting....they were also protecting Bree-country. I have the impression that since the Rangers were descended from the folks of the northern kingdom that they were just there still protecting the kingdom. How about this? They gave the Shire special protection because Gandalf suspected the ring might be the One but at least knew that it was a ring of power.