AB: “But I’ll give you a much more-likely one: Some 5-rate dictatorship somewhere in the world passes legislation (or, more likely: issues a decree) that the grandchildren or even great-great-grandchildren of its subjects are its own NBCs regardless of place of birth.”
Your supposition above make no difference since it was legislation of another country. US Law does not honor the laws of other countries over our citizens. That is the very basis of citizenship. But US law does acknowledge that within our borders there are aliens. Our laws do acknowledge that these non-citizens are subject to the laws of their own heritage. Until they are full US citizens, these people are still under the authority of their “fatherland”.
But as Blue Wall suggests (and I concur) is that Congress has authority to define citizenship (without amendment) as it pertains to those born on foreign soil and/or with dissimilar parentage. But Congress has no authority to change the status of those who are natural born. It seems obvious to me that if Congress could pass laws revoking the citizenship of an individual, it must mean that same individual does not have the Constitutional protection of a NBC.
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Not quite true. The exception is the case of dual citizens.
"Each country has its own nationality laws based on its own policy. Persons may have dual nationality by automatic operation of different laws rather than by choice."...The US Government recognizes that dual nationality exists but does not encourage it as a matter of policy because of the problems it may cause. Claims of other countries on dual national U.S. nationals may conflict with US law, and dual nationality may limit US Government efforts to assist nationals abroad. The country where a dual national is located generally has a stronger claim to that person's allegiance.
However, dual nationals owe allegiance to both the United States and the foreign country. They are required to obey the laws of both countries. Either country has the right to enforce its laws, particularly if the person later travels there."