The Constitution of the United States, to wit, Article III
Section 3. Treason against the United States, shall consist only in levying war against them, or in adhering to their enemies, giving them aid and comfort.
Using the strict interpretation of that section of the Constitution, I suspect that firing on a United States Army Fort by South Carolinians, or leading an army from Northern Virginia to invade Pennsylvania, could be considered treason in some circles.
What gave the north ownership of the South such that they could FORCE them to remain in a union where they were being financially raped by northern interest? The South left, peacefully. That wasn't good enough for the northern financiers who were profiting from the slanted trade and tariff rules resulting in one fourth of the population paying 90 percent of the funding for the federal government. Your monster government today is who won the war. Be proud.