Revolutionizing Citizenship of the United States
Faced with the defeated South poised to regain voting rights and seats in Congress, the Republican party desperately needed dependable voters and it created a new category of black citizenship to attain this.  Armed with a newly enacted law, the terrorist wing of the party, the Union League, was unleashed on the South with instructions to enlist freedmen and intimidate white voters from the polls.
Bernhard Thuersam, Chairman
North Carolina War Between the States Sesquicentennial Commission
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Revolutionizing Citizenship of the United States
“ . . .

[T]he provision of the [Civil Rights] bill [of 1866] appeared out of all relation to our constitutional system. Never before had Congress been known to arrogate to itself the power to regulate the civil status of the inhabitants of a State . . . , [the jurisdiction of disputes relating to contracts and property and even criminal actions, and it] seemed like a complete revelation of that diabolical spirit of centralization, of which only the cloven hoof had been manifested heretofore.
In addition to the definition of “slavery” and “involuntary servitude,” the Civil Rights Bill undertook to fix the precise meaning of the phrase “citizen of the United States.” For general practical purposes, exact determination of the scope of citizenship had not been found necessary [previous to the War].  Where any opinion at all had been pronounced, it had in most cases been in relation to the status of free Negroes.
The weight of authority on this point was adverse to the claim of citizenship for the blacks.  “No person,” said Attorney-General [William] Wirt in 1821, “is included in the description of citizen of the United States, who has not the full rights of a citizen in the State of his residence.”
This principle had been in general the basis of the government’s practice in all the departments. For native-born persons living within a State, citizenship of the State was the prerequisite for citizenship of the United States; for persons of foreign birth, naturalization alone was necessary. The Dred Scott decision limited this rule by determining that State citizens of African descent could not be citizens of the United States.
During the war, however, the old view was entirely overthrown in practice.  Mr. Lincoln’s attorney-general argued away all the precedents, and gave it as his official opinion that a free Negro, born within the United States, was ipso facto a citizen thereof.  He assumed nativity as the broad basis of citizenship . . . [and] With that assumption the status of United States citizenship was placed entirely beyond the reach of any State influence whatever, and a purely national conception was attained.
To justify this sweeping enactment, the special conception of citizenship which the history of our institutions had developed was discarded, and the broad principle of public law was adopted in its place.  All [Northern] authorities agreed that the status of citizen implied the reciprocal duties of allegiance and protection.  A citizen of the United States, then, was entitled to the protection of that government to which allegiance was owed. But this protection was to operate against all sources of oppression, and if a State government happened to come in this category, it too must succumb.”
(Essays on the Civil War and Reconstruction and Related Topics, William Archibald Dunning, The MacMillan Company, 1898, pp. 96-99)