Realm Divide, talk about a challenge.

[TinCow]

I found it was crucial to get your economy on solid ground before Realm Divide. You need to be prepared to operate for the rest of the game without ANY trade income. If you manage to keep some, consider it a bonus, but make sure you can fund your armies without it. If your economy collapses during Realm Divide, you're toast.

Another strategy I used successfully is to break up AI alliances and provoke wars prior to Realm Divide. The AI will break alliances if you pay them enough to do it, so if there is an AI alliance that you do not want to face during Realm Divide, use diplomacy to break it, and possibly even get them to declare war on one another. The more AI v AI fighting there is at the time of Realm Divide, the easier your conquest will be.