Really?! First of all, the federal government was created by the several States. It only had the authority that was delegated to it. Our federal ( not National ) Constitution is a compact amongst individual Sovereign entities. Webster understood this fact when he said:
If the South were to violate any part of the Constitution intentionally and systematically, and persist in so doing, year after year, and no remedy could be had, would the North be any longer bound by the rest of it? And if the North were deliberately, habitually, and of fixed purpose to disregard one part of it, would the South be bound any longer to observe its other obligations? I have not hesitated to say, and I repeat, that if the Northern States refuse, willfully and deliberately, to carry into effect that part of the Constitution which respects the restoration of fugitive slaves, and Congress provide no remedy, the South would no longer be bound to observe the compact. A bargain cannot be broken on one side and still bind the other side.
As did James Buchanan:
The question fairly stated is: Has the Constitution delegated to Congress the power to coerce a state into submission which is attempting to withdraw or has actually withdrawn from the confederacy? If answered in the affirmative, it must be on the principle that the power has been conferred upon Congress to declare and to make war against a state. After much serious reflection, I have arrived at the conclusion that no such power has been delegated to Congress or to any other department of the federal government. It is manifest upon an inspection of the Constitution that this is not among the specific and enumerated powers granted to Congress, and it is equally apparent that its exercise is not “necessary and proper for carrying into execution” any one of these powers. So far from this power having been delegated to Congress, it was expressly refused by the Convention which framed the Constitution.
Journal of the Federal Convention, Vol. 1, May 31, 1787:
Mr. MADISON observed, that the more he reflected on the use of force, the more he doubted the practicability, the justice and the efficacy of it, when applied to people collectively, individually. Any union of the States containing such an ingredient seemed to provide for its own destruction. The use of force against a State would look more like a declaration of war than an infliction of punishment; and would probably be considered by the party attacked as a dissolution of all previous compacts by which it might be bound.
“f the Northern States refuse, willfully and deliberately, to carry into effect that part of the Constitution which respects the restoration of fugitive slaves, and Congress provide no remedy, the South would no longer be bound to observe the compact”
Wait...wait! I thought it was all about taxes...
Again, your claim was that nobody thought that secession was illegal. You are incorrect in that.
Webster understood this fact when he said...
Webster also said:
"I should much prefer to have heard from every member on this floor declarations of opinion that this Union could never be dissolved, than the declaration of opinion by any body, that, in any case, under the pressure of any circumstances, such a dissolution was possible. I hear with distress and anguish the word "secession," especially when it falls from the lips of those who are patriotic, and known to the country, and known all over the world, for their political services. Secession! Peaceable secession! Sir, your eyes and mine are never destined to see that miracle. The dismemberment of this vast country without convulsion! The breaking up of the fountains of the great deep without ruffling the surface! Who is so foolish, I beg every body's pardon, as to expect to see any such thing? Sir, he who sees these States, now revolving in harmony around a common centre, and expects to see them quit their places and fly off without convulsion, may look the next hour to see heavenly bodies rush from their spheres, and jostle against each other in the realms of space, without causing the wreck of the universe. There can be no such thing as peaceable secession. Peaceable secession is an utter impossibility. Is the great Constitution under which we live, covering this whole country, is it to be thawed and melted away by secession, as the snows on the mountain melt under the influence of a vernal sun, disappear almost unobserved, and run off? No, Sir! No, Sir! I will not state what might produce the disruption of the Union; but, Sir, I see as plainly as I see the sun in heaven what that disruption itself must produce; I see that it must produce war..."
As did James Buchanan...
From the same speech:
"The right of the people of a single State to absolve themselves at will and without the consent of the other States from their most solemn obligations, and hazard the liberties and happiness of the millions composing this Union, can not be acknowledged. Such authority is believed to be utterly repugnant both to the principles upon which the General Government is constituted and to the objects which it is expressly formed to attain. It is not pretended that any clause in the Constitution gives countenance to such a theory."
Mr. MADISON observed, that the more he reflected on the use of force, the more he doubted the practicability, the justice and the efficacy of it, when applied to people collectively, individually. Any union of the States containing such an ingredient seemed to provide for its own destruction. The use of force against a State would look more like a declaration of war than an infliction of punishment; and would probably be considered by the party attacked as a dissolution of all previous compacts by which it might be bound.
Nothing in that says anything about secession or how is should, or could, be accomplished. Madison is on record as saying that a lawful secession requires the consent of all the states. The Southern states did not try that route when they chose to leave.