Attorney General Black’s Opinion of November 20, 1860
Faced with a growing secession crisis President James Buchanan asked his Attorney-General Jeremiah S. Black for information regarding his legal powers. He advised Buchanan that he could mobilize militia to achieve enforcement of federal laws under the Act of 1795 which provided that the militia could be called “whenever the laws of the United States have been opposed…by any State, by any combinations too powerful to be suppressed by the ordinary course of legal proceedings.”
Buchanan then realized that he would first have to move troops against Massachusetts, Wisconsin, and the other Northern States whose Personal Liberty Laws had undermined enforcement of the Federal Fugitive Slave Act.  These States had committed overt acts of defiance of federal law – nullification – as yet South Carolina had not.  Buchanan could not march against a people who only talked against the federal government, ignoring States which had for years been actively obstructing the course of federal law.  If one holds the belief that the Southern States, despite secession, remained part of the union, then the federal government was Constitution-bound to protect them; e.g., South Carolina against troops from New England States,
Bernhard Thuersam, Chairman
North Carolina War Between the States Sesquicentennial Commission
"The Official Website of the North Carolina WBTS Sesquicentennial"
Attorney General Black’s Opinion of November 20, 1860:
“Whether Congress has the constitutional right to make war upon one or more States, , and require the Executive of the Federal Government to carry it on by means of force  to be drawn from the other States, is a question for Congress itself to consider.
It must be admitted that no such power is expressly given; nor are there any words in the Constitution which imply it.  Among the powers enumerated in Article 1, Section 8, [it] certainly means nothing more than the power to commence and carry on hostilities against the foreign enemies of the nation.
Another clause, in the same section gives Congress the power “to provide for calling forth the militia,” and to use them within the limits of the State. But this power is also restricted by the words which immediately follow that it can be exercised only for one of the following purposes: To execute the laws of the Union;….To suppress insurrections against the State; but this is confined by Article 4, Section 4, to cases in which the State itself shall apply for assistance against her own people.
[And] 3. To repel the invasion of a State by enemies who come from abroad to assail her in her own territory.  All these provisions are made to protect the States, not to authorize an attack by one part of the country by another; to preserve the peace, and not to plunge them into civil war.  Our forefathers do not seem to have thought that war was calculated “to form a more perfect Union, establish justice, insure domestic tranquility, provide for the common defense, promote the general welfare, and secure the blessings of liberty to ourselves and our posterity.”
There was undoubtedly a strong and universal conviction among the men who framed and ratified the Constitution, that military force would not only be useless, but pernicious, as a means of holding the States together.
If it is true that war cannot be declared, nor a system of general hostilities carried on by the Central Government against a State, then it seems to follow that an attempt to do so would be ipso facto an expulsion of such State from the Union. Being treated as an alien and enemy, she would be compelled to act accordingly.
And if Congress shall break up the present Union by unconstitutionally putting strife and enmity and armed hostility between different sections of the country, instead of the domestic tranquility which the Constitution was meant to insure, will not all the States be absolved from their Federal obligations?”
(Twenty Years of Congress, James G. Blaine, Volume I, Henry Hill Publishing Company, 1884, pp. 604-605)