Not it was not. The Supreme Court ruled so in 1869.
Legal Secession Conventions were conducted in accordance with the Constitution.
The Constitution is silent on the method of secession, so stating that secession conventions were conducted in accordance with the Constitution would be incorrect.
Several of these States have combined to form a new Confederacy, claiming to be acknowledged by the world as a Sovereign State … Their right to do so is now being decided by wager of Battle."
Please continue with the quote. The Court went on to say:
"The ports and territory of each of these States are held in hostility to the General Government. It is no loose, unorganized insurrection, having no defined boundary or possession. It has a boundary marked by lines of bayonets, and which can be crossed only by force—south of this line is enemies’ territory, because it is claimed and held in possession by an organized, hostile and belligerent power.
All persons residing within this territory whose property may be used to increase the revenues ofthe hostile power are, in this contest, liable to be treated as enemies, though not foreigners. They have cast off their allegiance and made war on their Government, and are nonetheless enemies because they are traitors."
The Court is recognizing the southern acts for what they were, a rebellion of the states against their government.
The CSA did in fact legally secede from the Union. The problem was that the North refused to recognize it due to the fact that they were going to lose all that revenue from those high tariffs they had been collecting.
Losing the small percentage of the tariff generated by the southern states would have hurt, but not significantly. That was hardly a reason for war.
Not it was not. The Supreme Court ruled so in 1869."
Please explain how a supreme court decision made in 1869 could be used as justification to invade a sovereign nation in 1861, Eight (8) years after the fact.