You must remember that the States freely associated with each other to form the united States (little u). The Constitution is the framework by which the division of powers was formulated between the Federal government, State governments, and the people. If the Constitution did not specifically enumerate a power of the Federal government, IT DID NOT HAVE THAT POWER!
The States were sovereign and independant entities, and were free to legislate and regulate to theirs hearts content, as long as they did not usurp powers that they agreed would be the province of the Federal Government or of the people.
The Federal government took on a national character when dealing with forces external to the states, ie - diplomacy and war. The federal character was invoked in internal dealing, with powers strictly limited by the Constitution.
The people were citizens of their State first and the united States second. For internal matters, State citizenship was paramount, and National citizenship took hold in external dealing, same as with the Federal government.
Each State is supposed to be free to chart it's own destiny, adhere to it's own beliefs, and formulate the character of the State, regardless of what the other States feel or do, and regardless of what the other States think of what they are doing. If a citizen of the State does not like living in the society and adhereing to the laws of his State, he can vote with his feet and move to another State. If enough people don't like the Status Quo they can vote the bastards out of office and make the desired changes through their newly elected representative.
There is no restriction in the Constitution to keep the States from seceeding from the uniited States. More specifically, there is no power enumerated in the Constitution to empower the Federal government to stop a State from seceeding.
The Federal government, dominated by Nothern States power block, had been levying increasingly excessive protectionist tariffs and duties for at least 50 years before 1861. The tariffs and duties were enacted in an effort to build up the industrial infrastructure of the Northern States, and most of the tariff and duties were paid by the agricultural Southern States.
After decades of attempting to alleviate the financial drain placed on the Southern economies by the Federal tariffs and duties, the South decided in 1861 to vote with their feet, and leave the Union.
After secession the Federal government initially did nothing to nullify the secession and "preserve" the Union. President Buchanan refused to send troops to put down the "insurrection" because he could find no Constitutional authority that would permit him to do so legally. Abraham Lincoln ignored the Federal government's enumerated powers and precipitated the War of Northern Agression to placate the Northern States, who were themselves threatening to seceed from the Union so they could establish free trade ports that could compete with the new Southern free trade ports.
The long term end results of the War of Northern Agression was to futher erode and eliminate the rights of the individual States to determine the character and nature of their own society. This has continued to this day as the power of the States continues to be usurped with every new unconstitutional law passed by the Federal government. What does that mean to you?
http://www.snowcrest.net/siskfarm/nullif.html
"TARIFF OF ABOMINATIONS"
(Sources: Davisdon, Gienapp, Heyrman, Lytle and Stoff's Nation of Nations, A Narrative History of the American Republic, Alfred A. Knopf, c1991; and Alfred H. Kelly & Winfred A. Harbison, The American Constitution, Its Origins and Development, Fourth Edition, W.W. Norton & Co., Inc. c1970.)
Congress had raised duties in 1816 and again in 1824. The tariff of 1824 included high duties on imported agricultural goods such as hemp, wheat and liquor to protect western farmers; imported textiles to protect New England interests; and iron to protect mining and forging industries of Pennsylvania. South Carolina had been particularly hard hit by the depression of 1819. The tariffs increased the prices of their imported goods by as much as 50 percent. South Carolina asserted that the tariffs were unfair as a tax on Southern agriculture for the benefit of Northern industry.
When Congress raised the duties even higher in 1828 with the so-called "Tariff of Abominations," South Carolina's Legislature published the "South Carolina Exposition and Protest," or South Carolina Doctrine, protesting the tariff as unconstitutional and advancing the theory of nullification. The Exposition declared that a sovereign State had the right to determine through a convention whether an act of Congress was unconstitutional and whether it constituted such a dangerous violation "as to justify the interposition of the State to protect its rights." If so, the convention would then decide in what manner the act ought to be declared null and void within the limits of the State, and the declaration would be obligatory on her own citizens, as well as the national government.
U.S. Vice President John C. Calhoun was the (secret) author of the nullification theory. The Union, he argued, was a compact or league between sovereign States. Sovereignty by its very nature was indivisible and absolute. Ultimate sovereignty could not be inherent in both the federal and State governments. The American colonies had always existed as distinct political communities, which became free, independent and sovereign states at the revolution. The Articles of Confederation had recognized that separate sovereignty. The Constitution of the United States had also been drafted by delegates acting and voting as states and the compact had been ratified by the separate states, each acting as a sovereign entity. Although the various States had delegated a portion of their functions to the federal government, they had not surrendered their ultimate sovereignty, which was by nature indivisible.
The Constitution was not supreme law, but a contract or agreement between sovereign states that set up a federal government to perform certain functions for the contracting parties. As such, the States possessed the final authority to interpret the Constitution. The central government could not pretend to sovereignty. There was no such thing as an American nation. The "people" were a political fiction, which under central government had come to mean the collective popular majority. The numerical majority would become tyrannical and disregard the Constitution in order to destroy minority rights. The only safeguard for minority rights was State sovereignty and nullification. Nullification was "simply a declaration on the part of the principal, made in due form, that an act of the agent transcending his power is null and void." (The American Constitution at 309.)
The central government was not a separate sovereignty, but simply an agent of the several States. Thus the people of each State, acting in special conventions, had the right to nullify federal law that exceeded the powers granted to Congress through the Constitution. If a popular convention declared a law unconstitutional, it would become null and void in a State. Congress could then either yield and repeal the law or propose a constitutional amendment expressly giving it the power in question. If the amendment was ratified by three-fourths of the States and added to the Constitution, the nullifying State could then either accept the decision or exercise its ultimate right as a sovereign state and secede from the Union.
When Senator Robert Hayne of South Carolina explained Calhoun's theory on the floor of the Senate, Senator Daniel Webster of Massachusetts replied that the Union was not a compact of sovereign states.
When Congress passed another tariff in 1832, moderating some of the duties of the prior act but continuing the protective system, South Carolina's legislature called for the election of delegates to a popular convention on November 10 to decide whether the State would nullify the new tariff according to Calhoun's formula. The convention overwhelmingly adopted an ordinance of nullification drawn by Chacellor William Harper by a vote of 136 to 26. The ordinance declared the tariffs of 1828 and 1832 "unauthorized by the Constitution" and "null, void, and no law, nor binding upon this State, its officers or its citizens" after February 1, 1833. The convention also established legal penalties for any State or federal officer who attempted to collect the tariff duties. It declared that in no case at law or equity in the courts of the State could the validity of the ordinance or implementing legislation be questioned and that no appeal could be taken to the Supreme Court of the United States.
The legislature further enacted a replevin act authorizing the owner of imported goods that were seized by customs officials for nonpayment of duties to recover them (or twice their value) from customs officials. The law also authorized the governor to call out the militia to enforce the laws of the State. The nullifiers declared that any effort of the federal government to employ naval or military force to coerce the State, close its ports, destroy or harass its commerce, or enforce the tariff acts, would impel the people of South Carolina to secede from the Union and organize a separate independent government.
In November 1832, President Andrew Jackson sent seven small naval vessels and a man-of-war to Charleston with orders to be ready for instant action. In December, 1832, he issued a "Proclamation On Nullification" to the people of South Carolina written by Secretary of State Livingston, stating that the Union was perpetual, and under the Constitution, there was no right of secession. The United States was a nation and not a league and secession was revolutionary and would destroy the nation.
The power to annual a law of the United States, assumed by one State, Jackson stated, was "incompatible with the existence of the Union, contradicted expressly by the letter of the Constitution, unauthorized by its spirit, inconsistent with every principle on which it was founded, and destructive of the great object for which it was formed," (Robert Kelley's The Shaping of the American Past, Fifth Edition, Prentice Hall, c1990, pg 266.)
The proclamation ended in a strong plea and threat: "Those who told you that you might peaceably prevent [the execution of the laws] deceived you; they could not have been deceived themselves... Their object is disunion. But be not deceived by names. Disunion by armed force is treason. Are you really ready to incur its guilt? If you are, on the heads of the instigators of the act be the dreadful consequences; on their heads be the dishonor, but on yours may fall the punishment. On your unhappy State will inevitably fall all the evils of the conflict you force upon the Government of your country... I adjure you ... to retrace your steps." (Source: Essay by Hal Morris based mainly on: Robert V. Remini's, The Life of Andrew Jackson for The American Revolution.)
Jackson claimed he could have 100,000 men on the side of the Union in a matter of weeks. The South Carolina legislature authorized its Governor to call a draft, and appropriated $200,000 for arms.
On the 16th of January, Jackson requested that Congress take steps that would "solemnly proclaim that the Constitution and the laws are supreme and the Union indissoluble." (The American Constitution at 313.) He also sent to Congress the "Force Bill" (often called the "Bloody Bill"), reaffirming the President's powers to call up State militias, the army and navy to quell any insurrection and granted greater powers to use the courts to enforce collection of duties. The bill asserted the supreme sovereignty of the national government and its right to enforce its statutes directly upon individuals by force, if necessary. The Force Act passed in the Senate 32-1, with nearly all the nullifiers having walked out to avoid casting any vote and in March, the House passed the Force Act 149-48. On March 1, the Senate passed a Compromise Tariff.
The nullifiers reassembled their popular convention on March 11 and repealed the nullifying ordinance, then proceeded to nullify the Force Act. (Jackson chose to ignore the latter.)
Obviously you've given some time to studying the subject, so I don't need to tell you that it is somewhat complicated, since both sides contradicted themselves from time to time, probably on the basis of changing circumstance and quality of the arguments being presented. For instance, why would the states have to vie for readmission if they had no authority to separate in the first place? Why would the south claim it isn't about slavery, yet in many declarations of independence cite slavery as a central issue? Why would the south deny any claim to perpetuality of the union yet explicitly assume it in their Articles of Confederation?
Likewise, it's too simplistic to claim stats as sovereign. Of course they were subject to the laws of the federal/national government. The Constitution allowed for a sort of mixed sovereignty. Part rested with the federal government, part with the states, and part with the people. That in no way implies complete sovereignty or complete independence of the states.
Buchanan didn't send reinforcements, and neither did Lincoln, but he refused to acknowledge any right of the states to secede. His main motivation was to NOT be the president under whom the union dissolved. That makes him weak in character, not a wise sage showing noble restraint.
As to the so-called "Northern Aggression," keep in mind South Carolina fired first. They attacked federal property, attained under valid and mutually respected laws. The fact South Carolina claimed to have validly left (just as they fallaciously claimed the ability to nullify federal laws) the union, does not change the fact that it was federal property on which they fired. Congress has the power, vested by the Constitution, "To exercise exclusive Legislation in all Cases whatsoever, over such District (not exceeding ten Miles square) as may, by Cession of particular States, and the Acceptance of Congress, become the Seat of the Government of the United States, and to exercise like Authority over all Places purchased by the Consent of the Legislature of the State in which the Same shall be, for the Erection of Forts, Magazines, Arsenals, dock-Yards, and other needful Buildings;"
Needless to say they were empowered to protect federal property, as well as quell rebellions, and protect each state against domestic violence. These were powers vested in the federal government, and it likewise had the authority to make laws necessary for carrying out the powers granted in the Constitution, which included the above. To claim it was a war of "Northern Aggression," then, is specious when we consider that South Carolina fired first on federal property. Part of the union or not, the property was properly attained, and the federal government's claim to it was not denied to it in open court. Therefore the attempt to sieze it by force was either an act of rebellion or an act of aggression.