After pretending to an illegal and unconstitutional secession the southern insurrection opened fire on US forces at Ft. Sumter, and declared war. That, as most honest people recognize, made legal considerations moot.
Grant was many things, but he was not a lawyer. Nor was Lee.
The legal opinion is Texas v. White. I recommend you look it up. It uses short words, so you will be able to follow it.
Seems like you disagree with Grant and Eisenhower. You are losing historical friends fast.