"I cannot imagine that such a question could ever reach the Supreme Court,” Scalia wrote. “To begin with, the answer is clear. If there was any constitutional issue resolved by the Civil War, it is that there is no right to secede. … Secondly, I find it difficult to envision who the parties to this lawsuit might be. Is the state suing the United States for a declaratory judgment? But the United States cannot be sued without its consent, and it has not consented to this sort of suit."Scalia's comments address two critical questions. First, he quite astutely points out that as a practical matter, a state simply does not have the right to unilaterally secede from the Union. Second, and I think more interestingly, he addresses the question of the justicability of a claim by a state to have such a right. As he points out, there simply is no mechanism for such a claim to be brought by a state in court. The claim is not only extra-constitutional, it is extra-legal. As a consequence, it is not in any meaningful sense, a legal claim.
Scalia's points are absolutely damning when it comes to assertions of a legal right to secession by a state. Even if it could be established that a state had the constitutional right to secede, the state still could not make a legal claim to that effect because it is unable to bring suit against the federal government without the federal government's consent. The state could make a political claim, a moral claim, a philosophical claim. But it could not make a legal claim, because the state's case could not be placed before a court of law. Unless, of course, the federal government consented. But then the state would probably not be acting unilaterally in pursuit of secession.