“The fact your overlooking is that it was THE SOUTH that belonged and still belongs to the UNION”
Once the south had withdrawn from the union it then became CSA land, just like after the revolutionary war with england this became our land.
The United States of America had to earn it's freedom from England before we could truly call this land ours. That happened when Cornwallis surrendered to Washington at Yorktown. Just like England after the Declaration of Independence, The Union refused to recognize the CSA as a sovern nation when the CSA declared it's freedom.