It's not a debate about the definition of "invasion," it's just that the idea of "Confederates=defenders, Union=invaders" is worded to make it easier to sympathize with the Confederacy, when it does not accuately reflect the reality of the situation. And even if you did want to turn this into a debate about semantics, is it an "invasion" when the police break into a criminal organization's hideout? Because this is the equivalent, on a much larger scale. The Confederates were criminals/traitors, not an enemy force. They wanted independence, but they were not independent yet, and never became independent.