Sorry if this topic was already discussed.
Well, in the first Victoria (with revolutions) it was very annoying when the AI was making a separate peace in a war. I remember a (VIP) game as Bavaria, when I organized an anti-Prussian coalition (Prussia had won the Austria-Prussian war), consisting of me, Austria, France and Russia vs. Prussia and the Ottoman Empire. Well, The French were (very slowly) pushing in the west, Austria-Russian forces were almost in Berlin, the Turkish front was being easily won, and then.... Austria made peace with Prussia (demanding 1-2 provs). The eastern front slowly collapsed, separate peace with Russia, well, the rest is obvious.
I remember an common problem was discussed in the HoI3 thread, although there it was mainly on the problem that you couldn't launch an cooperative attack with an allied country.