Tocqueville put it best in one of those many pre-war political commentaries most Lincoln worshipers and Claremonsters like to overlook...
"However strong a government may be, it cannot easily escape from the consequences of a principle which it has once admitted as the foundation of its constitution. The Union was formed by the voluntary agreement of the states; and these, in uniting together, have not forfeited their sovereignty, nor have they been reduced to the condition of one and the same people. If one of the states chose to withdraw its name from the contract, it would be difficult to disprove its right of doing so, and the Federal government would have no means of maintaining its claims directly, either by force or by right."
"However strong a government may be, it cannot easily escape from the consequences of a principle which it has once admitted as the foundation of its constitution. The Union was formed by the voluntary agreement of the states; and these, in uniting together, have not forfeited their sovereignty, nor have they been reduced to the condition of one and the same people. If one of the states chose to withdraw its name from the contract, it would be difficult to disprove its right of doing so, and "
I'm glad you brought up Tocqueville, since he's my favorite political analyst. As perceptive as he was, however, he was not infallible, and clearly he was wrong in assuming that "the Federal government would have no means of maintaining its claims directly, either by force or by right". And of course, the U.S. never had to disprove the Confederate states' right to withdraw from the Union, since the seceding states never attempted to prove any right under the Constitution to secede.
But if you're truly enamored with Tocqueville, I take it that you'd agree with some of his other assertions in Democracy in America, such as the following:
"The legislation of the Southern states with regard to slaves presents at the present day such unparalleled atrocities as suffice to show that the laws of humanity have been totally perverted...In antiquity precautions were taken to prevent the slave from breaking his chains; at the present day measures are adopted to deprive him of even the desire for freedom...the Americans of the South, who do not admit that the Negroes can ever be commingled with themselves, have forbidden them, under severe penalties, to be taught to read or write; and as they will not raise them to their own level, they sink them as nearly as possible to that of the brutes." Democracy in America (Vintage Books Ed. 1990), Vol. 2, pp. 379-380.
"The Americans have ... much more ... to fear from the states than from the Union." DIA, p. 385.
"In the South there are no families so poor as to not have slaves. The citizen of the Southern states becomes a sort of domestic dictator from infancy; the first notion he acquires in life is that he is born to command, and the fist habit which he contracts is that of ruling without resistance. His education tends, then, to give him the character of a haughty and hasty man, irascible, violent, ardent in his desires, impatient of obstacles, but easily discouraged if he cannot succeed upon his first attempt." DIA, Vol. 2, p. 394.
"The inhabitants of the Southern states are, of all Americans, those who ... would assuredly suffer most from being left to themselves; and yet they are the only ones who threaten to break the tie of confederation." DIA, Vol. 2, p. 401.
"If ever America undergoes great revolutions, they will be brought about by the presence of the black race on the soil of the United States; that is to say, they will owe their origin, not to the equality, but to the inequality of condition." DIA, Vol. 1, p. 256.