People keep saying this, but I don't think it really makes much sense gameplay wise or realistically wise.
Think of it like this. Texas willingly joined the US. Now, for whatever reason, Mexico captures Texas from the US and says be free Texas, go back to being the Texas Republic.
Texas reply, "Umm yeah we are a part of the US...we will just be going back now"
Gameplay wise it doesn't make sense either. How much worse would Austria and Venice be if I could go on a "conquering spree" and "liberate" all of their cities, which they didn't take by force? So now the world sees me not as a warmonger who just destroyed nearly all of Venice, but instead as a liberator of the ones who needed no liberation?