Ending other faction's wars through your diplomacy was quite common in RTW (though ceasefire didn't last).

What's interesting in MTW2 is that now you can keep allies even if they are waging war against each other .

I dunno if it should really work like this - but I'd like to see what happens when you get dragged into a battle of your allies (you know - when your stack is close to others' battles, only that now you're allied with both of them).