Secession is legal, if the consent of the other states is gained and expressed through amendment or through Congress.
Absent that it is not secession but rather insurrection, and I have no doubt that the mismatch between US military power and any insurrection would be greater now than in 1865.
NE states will never agree to the land and oil rich states leaving and taking their assets with them.
>>>I have no doubt that the mismatch between US military power and any insurrection would be greater now than in 1865
That may be true. But can the federal government get away with large-scale killing of people in the television, mobile phone, and youtube era?