The Realm Divide

The Realm Divide is triggered on three different occasions:

1. Gaining a legendary fame with the current Shogunate by taking more than 15-18 provinces (number depends on difficulty)
2. Taking Kyoto in the first place
3. Being announced Shogun (after holding Kyoto for 4 seasons)

All three types of the Realm Divide trigger a penalty for all clans towards you. That means, making vassals or allies during each of these phases will cause a new penalty.
If you become Shogun before you hold the maximum provinces, then point one becomes obsolete. Taking Kyoto raises your fame to legendary immediatly and there is no going back.
The malus will stop counting at -200 against you and that will probably cause even your longest ally to turn against you in the end. New clans (or rerised) will not suffer of the Realm Divide penalty, but still tend to side against you, although your standing was "friendly".

So triggering the Realm Divide should be chosen wisely and needs a lot of preparation.

Note: If an AI clan usurpate the shogunate it will not suffer any penaltys and its allies will still be there