at the same time? I'm playing as California with the expansion pack. Mexico started off by declaring war on different states. I declared war on them when I had a lot of Cassus Belli towards them. It angered many countries, but their opinion of me changed back to normal pretty quickly. I took all of Baja California from Mexico, which was quite easy. At the same time, Texas took a lot of Mexico around itself. I decided to start taking more land from Mexico, heading towards its capital. At this time, Arizona decided it would be cool to declare war on Mexico as well. Arizona cut me off from Mexico in the process. I decided that I wasn't going to have any of this, so I declared war on Arizona. They weren't very liked after their little stunt, and they were even expelled from the U.N. So, I thought I wouldn't have any repercussions from my declaration. Well, almost the entire world hates me and the U.N. expelled me; despite them not liking Arizona. So, how do I get everybody to like me again quickly?
Make peace with Arizona and give the wound time to heal. By declaring on a country without 80 or 90% or better CB you have invoked global wrath. You would have done better provoking Arizona by making treaties with it and moving military forces to its border, then canceling all the treaties and letting them declare war on you. It would have given the world far less of a CB if done properly. When another nation declares war on you, it reduces other nation's CB's against you.