It proves, without a doubt, that secession is legal. You will not find this in any history book. Please feel free to post that link, post it often.
Basler, Vol. II, The Collected Works of Abraham Lincoln (1953), p. 115.
On January 12, 1848, Lincoln, in an address before the United States House of Representatives, stated:
Any people anywhere, being inclined and having the power, have the right to rise up, and shake off the existing government, and form a new one that suits them better. This is a most valuable — a most sacred right — a right, [p270] which we hope and believe, is to liberate the world.
The Supreme Court ruled that secession as practiced by the rebel states was not.