What does realm divide do in Shogun 2
Solution 1:
When you achieve enough fame, the shogun considers you a threat and orders all other clans to declare war. This includes allies who you may have had since the start of the game.
The mechanism by which this work is that all your relationships are set at -40 and continue to decrease as turns pass. Now, any new vassels you create will not be affected by this penalty (nor will their relationships).