You must not be American otherwise you'd know the answer to the (rhetorical?) questions. The American Civil War showed us the atrocities each side was willing to commit. It was warfare at a level the rest of the world wouldn't experience for decades to come.

And unlike the first Civil War, the United States didn't have the means to destroy the rest of the world should their weapons fall into the wrong hands. The world has an interest to concern itself and intercede. When you say the end of the empire is going to be bad, that's not the half of it - it's almost unthinkably bad.