They defended their homeland from invasion by a foreign power.
Hence the war’s proper name: The War of Northern Aggression.
So would that mean that the proper name for World War II would be The War of Polish Aggression?
Actually the most accurate name is the official name give by the US Army, the war of rebellion. The United States had a constitutionally mandated election for president of the United States. During this election there were no claims of malfeasance by anyone. However, some states attempted to influence the election by threatening to leave the Union if one candidate in particular was elected.
After the election was done, but even before the new president was inaugurated, some states decided to rebel and breakaway from the US. While breaking away they grabbed every piece of federal property they could. These states even sent envoys to other states to convince them to rebel. They also suppressed their own citizens that did not want to rebel.
They then, without provocation, fired on US forces and then declared war against the US. If that’s not rebellion and treason, I don’t know what is.