The good news: the effect of 'Realm Divide' is capped at -200, which means with a long-term trade agreement, an alliance, some chi arts, and a marriage, you can hold on to an ally or two. Provinces captured after Realm Divide will be more difficult to subdue (higher 'resistance to invaders'). Any clans not at least 'Indifferent' (anyone have a more exact number for this?) with you will declare war without further provocation you get a 'Clan joins your enemies' message. It causes an immediate drop of -50 in relations with everyone due to 'Realm Divide'. There is a mod to tone it down, but let's see what we know about the vanilla version.
