In Downes v. Bidwell, the Supreme Court concluded that the Citizenship Clause does not apply to people born in unincorporated territories of the United States.
Instead, if they had no other claim to US citizenship, they were considered non-citizen nationals of the United States. However, Congress later extended citizenship to such individuals if born in Puerto Rico, the Virgin Islands, Guam, or the Northern Mariana Islands.
Currently, non-citizen nationality at birth is provided to people born in American Samoa or Swains Island by 8 USC §1408.
The Foreign Affairs Manual gives historical background on non-citizen nationality and contains the following curious note:
The United States exercises sovereignty over a few territories besides those mentioned above. Under international law and Supreme Court dicta, inhabitants of those territories, (Midway, Wake, Johnston, and other islands) would be considered non-citizen U.S. nationals. However, because the INA defines "outlying possessions of the United States" as only American Samoa and Swains Island, there is no current law relating to the nationality of the inhabitants of those territories or persons born there who have not acquired U.S. nationality by other means.
Did the Supreme Court state in dicta that individuals born in unincorporated territories of the United States are constitutionally guaranteed US nationality, or was the Court simply making a remark about what they thought was the intent of Congress regarding the nationality of hypothetical individuals born in unincorporated territories not otherwise provided for in the Immigration and Nationality Act?