"The term resident and citizen of the United States is distinguished from a Citizen of one of the several states, in that the former is a special class of citizen created by Congress."
U.S. v. Anthony 24 Fed. 829 (1873)
Odd, I can't seem to find the ability to create citizenship in the Constitution.
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The areas 'subject to the jurisdiction of the United States' are quite specific, and can be found in Article 1, Section, Clause 17.
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People living in the enumerated areas are supposed to be US citizens because the are is the 'collective' aspect of the federal government not in the individual aspect of the States.
§ 1218. The inhabitants enjoy all their civil, religious, and political rights. They live substantially under the same laws, as at the time of the cession, such changes only having been made, as have been devised, and sought by themselves. They are not indeed citizens of any state, entitled to the privileges of such; but they are citizens of the United States. They have no immediate representatives in congress.
Joseph Story, Commentaries on the Constitution
IMHO, the federal government uses the 14th Amendment to turn the law of the Constitution inside-out by forcing everyone to be a 'US citizen' and refuse to even acknowledge that State citizens exist.
Not to mention the really handy ability to be the only judge of its own power.
Southern states that ratified the 13th months before were prohibited by yankees to seat their Congressmen. More than enough states rejected the amendment for it to fail, and yet it was "proclaimed" to have passed.