The Civil War has nothing to do with this, nor does it have to do with the US military being used against civilians.
The CSA was already established and had a standing army when THEY initiated hostilities against the Union. Therefore the military at the time wasn’t used against US citizens.
“The CSA was already established and had a standing army when THEY initiated hostilities against the Union. Therefore the military at the time wasn’t used against US citizens.”
Evidently you disagree with Abe Lincoln who said that there was no CSA. There were only American states in rebellion.
Lincoln called up troops to put down rebellion while Congress was out of session. He would have called Congress into special session to declare war if he had recognized the CSA, which he never did at any point.
The CSA saw things differently. Like you, they believed that they were a separate country. And as such they considered the presence of a rival military power in their territory to be unacceptable.
Lincoln apologists tend to forget that King George III considered the American colonists to be rebels who started a war against the legitimate government, one which incidentally offered freedom to blacks who supported the Crown. That complicates their desire to legitimize both the Founders and the Lincoln regime.