Tl;dr version of intro:
- 400 years ago Demon Wars in which Jinchuuriki were used as weapons. Resulted in agreement following those wars that they should not be used in that manner.
- 40ish years ago - Kazekage betrays the agreement, results in bijuu getting free and started rampaging around the mainland. They destroy a bunch of stuff and many of the nations merge to survive (see map). Bijuu are now stored in Shrines and are religious symbols for some scattered about the land. 1,2,9 are in Kyokujitsu no Tochi territory, 3,5,8 are in Land of Water. 6 and 7 are in Land of Earth. 4 is within Kyuueseishi territory.
- Currently, some smaller nations are less thrilled with being a part of larger nations.