I remember that there had been a prophecy that a Hobbit would defeat Sauron. Knowing this, the Rangers protected the Shire. Sauron probably knew the prophecy as well, so why didn't he ever try to attack the Shire and kill all the Hobbits so that this would never happen. I can tell it would probably be hard to get an army all the way from Mordor to the Shire, but wouldn't Sauron do everything in his power to stop his downfall?
Seek for the Sword that was broken:
In Imladris it dwells;
There shall be counsels taken
Stronger than Morgul-spells.
There shall be shown a token
That Doom is near at hand,
For Isildur's Bane shall waken,
And the Halfling forth shall stand.