Wargaming political negotiations is far less popular than wargaming, well, wars. You might be able to have a little fun with the first option -- the Union atta

Wargaming political negotiations is far less popular than wargaming, well, wars. You might be able to have a little fun with the first option -- the Union attacks first -- but it's still going to largely end up political questions: 1. If the Union attacked immediately after South Carolina seceded, how would it have changed which States would follow suit? 2. If the Union attacked in 1862, would more States have seceded by then? 3. Would the Union have lost any supporters -- either notable generals or even member States -- if it had fired first? After those political questions are answered you could have fun wargaming out the subsequent war with new sides, but trying to answer the political questions is not as easy or fun.