What section granted it?
The Constitution was to make the Articles of Confederate more perfect, and those Articles were perpetual.
When the States agreed to the Constitution, they knew it was a permanent deal and that was acknowledged even by its opponents, such as Patrick Henry.
The Tenth Amendment limits federal powers to those explicitly granted in the constitution. I have looked and looked and never seen anything to the effect that, if a state should secede, the fedgov may send the army to murder, rape, loot and pillage the civilian populations of such states. Where is that???
If a State attempts to secede, it is in violation to its obligations to the other states and in a state of Rebellion.
Now, since nothing has been done since to the Constitution to specifically forbid secession, you must still think that the right exists.
No government would give the right for parts of it to leave without a legitimate grievance.
That would not be a nation, that would be a coalition of sovereign nations, which the United States never was.
There was never a time when (with the exception of Texas) any State ever existed by itself and operated separately as a sovereign nation.
Even in the secession, it was a unified effort that immediately attempted to form another nation.
This act was expressly forbidden in Art.1, sec 10, the forming alliances with other states.
Now, if secession had been considered a viable option by the Founding Fathers, they would not have forbidden States to unite together against other states.
The CSA Constitution had the same restriction in it.
The states are now "protected" if at all be the people themselves of the respective states. Why this is not just as effective, if at all, as state legislative election of US Senators does not seem immediately obvious.
Not quite sure what you mean by this.
The reason that State Legislature's electing of Senators was an important defense of Federalism, was that the State Legislatures put the Senators in place as the State Representative.
Thus, the Senator always knew that he was representing that State.
Much like a Congressman is directly representing his own district. Direct Election of Senators removed that restriction and the Senators now can make direct appeals to the people of the State without taking the States best interest to heart.
It makes the Senate a more 'democratic' Body when it was to be the 'aristocratic' one, detached from the whims and shifting moods of the people.
Are you some sort of fan of Alexander Hamilton??? His Federalist Party was effectively dead before he was. Its successor, the Whig Party did not last long. The Republican Party came next and abandoned the Lincolnian tyranny soon enough to survive although the old Hamiltonian temptation is threatening the GOP yet again. Whenever the Hamiltonian impulse arises against the interest of the general public, the general public kills off the Hamiltonian Party du jour. "Suffer to the max so that the rulers may prosper personally" is not very salable politically in a democratic republic such as ours. AND, history is not easily amended to sanitize Hamiltonianism.
And Hamiltonianism is not so easily dismissed either.
Hamilton was concerned with the stability of the nation, and rightfully so.
The United States did not have to suffer through the traumas that racked the French and Russian revolutions.
Both Hamilton and Jefferson sought to protect individual liberty, but each simply leaned in a different direction, Hamilition toward stability which meant a stronger Federal Government.
As Jefferson himself stated in his first inaugural address,
But every difference of opinion is not a difference of principle. We have called by different names brethren of the same principle. We are all Republicans, we are all Federalists. If there be any among us who would wish to dissolve this Union or to change its republican form, let them stand undisturbed as monuments of the safety with which error of opinion may be tolerated where reason is left free to combat it.
http://www.bartleby.com/124/pres16.html
Now, to keep on message as to the constitution. Patrick Henry did not approve of the crushing of the states. He warned of it. He was an anti-federalist. The Tenth Amendment says what it means and means what it says. The default position established by the Tenth Amendent of the Bill of Rights makes clear beyond question that, the fedgov having no express constitutional right to attack militarily any state for dissolving the bonds that bind it to the "Union" (somewhat familiar language from another document, eh?), would violate the constitution by doing so. Secession being unreferenced as to any fedgov power specifically delegated by the constitution was left to the STATES and the PEOPLE respectively. Eleven states took that option and seceded (twelve if you count Wisconsin in 1859 "seceding" in protest and not wanting to be unified with slave states). In at least Virginia, the matter was ALSO popularly voted by the citizens.
Other than Lincoln's massacre of the constitution to preserve his power, NOTHING was ever done to the constitution, before or after the late unpleasantness, to deny to any state the right to secede.
It does not matter what the CSA constitution said as to secession or did not say since Lincoln, a resident first of Kentucky in his boyhood and thereafter in Illinois, never was subject to the CSA constitution.
Article I, Section 10 references "States" which, for purposes of the constitution, the seceded states ceased to be upon secession: Virginia, North Carolina, South Carolina, Georgia, Florida, Alabama, Mississippi, Louisiana, Tennessee, Texas and Arkansas. If they had not succeeded in seceding, Why would those states need to be readmitted to the "Union" with "conditions" no less like ratifying the 13th, 14th and 15th Amendments as "conditions of re-admission" and where did Congress get off imposing "reconstruction?" By 1877-1881, President Rutherford B. Hayes proved a far better president and a far better Republican than Lincoln by redeeming the greenback "fiat money" at FULL FACE VALUE, abolishing the unconstitutional personal income tax AND putting an end to the tyranny known as "reconstruction." I believe that he also balanced the budget and then went home to Ohio with a sense of having accomplished all that could be accomplished.