On the premise that the southern acts of unilateral secession were not acts allowed under the Constitution and therefore constituted rebellion. The Supreme Court agreed with that, BTW, in Texas v. White in 1869.
Could ANYONE have guessed what the conclusion reached by a northern, reconstruction era, court needing to justify the war of Northern Aggression might have been in Texas Vs White? Their answer was:
"The Constitution, in all its provisions, looks to an indestructible Union, composed of indestructible States. Considered, therefore, as transactions under the Constitution, the Ordinance of Secession, adopted by the convention and ratified by a majority of the citizens of Texas, and all the Acts of her Legislature intended to give effect to that ordinance, were absolutely null. They were utterly without operation in law. Our conclusion, therefore, is, that Texas continued to be a State, and a State of the Union, notwithstanding the rationale for military occupation is also self-contradictory."
Interesting enough in Coleman vs Tennessee, the U.S. Supreme Court held military occupation lawful, not on constitutional grounds, but by resorting to international law principles which apply primarily to independent nations.
"Though the late war was not between independent nations, but between different portions of the same nation, yet having taken the proportions of a territorial war, the insurgents having become formidable enough to be recognized as belligerents, the same doctrine must be held to apply. The right to govern the territory of the enemy during its military occupation is one of the incidents of war and the character and form of the government to be established depend entirely upon the laws of the conquering State or the orders of its military commander." (97 U.S. 509, 517; 1879)
It would appear then that to justify the otherwise unconstitutional military occupation of a state, the post war U.S. Supreme Court treats the state as if it were an independent nation, implicitly recognizing the validity of its secession. What the Court did not cite was any constitutional provision, which justified the war in the first place. Since the invocation of international law was based on the fact of war, and the Union's involvement in that war violated the Constitution, it is evident that the Constitution's supremacy clause forbade this action. Yet the Yankee government felt compelled to resort to international law to override the Constitution. The unconstitutional and amoral nature of the Court's reasoning can be seen.