Games : How do you avoid getting ganged-up on in Europa Universalis IV?

I decided to come back to this after a long time away because I got tired of it hanging on me. They seem to have fixed the hangs but every game I've played gets to a point where somebody attacks me and through a web of alliances (why should alliances even apply if they start the war??) I get everybody after me. Sure, they can't bite off too much in one go but once you've been smashed I see no way out of it.