Don't play on Very Hard... In my opinion it just makes it stupid and not hard... because the AI gets suicidally aggressive. On Normal, if you crush an AI faction's field army (And I mean crush, not just beat) it will generally send a diplomat post-haste to humbly request a ceasefire, and will generally accept an unfavorable agreement even.

On Very Hard they'll just keep scraping peasants together and throwing them at you until you destroy them completely.