The Constitution prohibits States from entering treaties, alliances, or confederations [Art I. Sec.10]. Although I found no pre-Civil War decisions on this subject, the plain text seems to make the Confederacy unconstitutional so that the seceding States could not be recognized as having any legal existence. Today, its practical significance lies in the limitations which it implies upon the power of the States to deal with matters having a bearing upon international relations.
Nullification Doctrine, first expressed in Kentucky and Virginia Resolutions of 1798 declaring Alien and Sedition Acts unconstitutional, is a legal theory that the Constitution is only a compact among states to delegate limited powers to a national government to deal with national affairs but not to interfere with the political, social, or economic structure or interests of the sovereign [self-governing] states. Therefore, states can ultimately determine whether a federal law exceeds authority of the national government, declare it unconstitutional, and refuse to enforce it. Jefferson drafted the resolutions, which advocate state rights and strict construction of Constitution. Calhoun used this theory to justify the Ordinance of Nullification of federal tariff laws in SC in 1832. Jackson responded with threat to deploy federal troops to enforce the law, stating that a state’s power to invalidate a federal law is inconsistent with the existence of a Union. The Constitution forms a government, not a league of sovereign states; and to say that a state may at will secede from the Union is to say the US is not a nation. [Proclamation on Nullification]
Confederate states adopted resolutions similar to Declaration of Independence to provide official justification for secession, comparing southern secession to the American Revolution. [Declarations of Secession]. These documents cite the Constitution [Art. IV] providing for return of fugitive slave. Recognition of slavery was essential to forming’’ a more perfect union’’ for southern states. South argued that northern anti-slave movement and failure to enforce fugitive slave provision justified secession. Northern states granted freedom to fugitive slaves by law [using nullification theory as justification]; refused to enforce Fugitive Slave Act [fed military very small]; encouraged Underground Railroad.