The Commonwealth of Puerto Rico occupies a strange netherworld within the United States. To quote an infamous Supreme Court decision from a century ago, it is "foreign in a domestic sense." Eminent scholars have suggested that the reason for giving Puerto Rico this unusual status was to preserve Congress's power to expel Puerto Rico from the Union. But is such a power plausible today? Puerto Ricans are natural born US citizens. Even if Congress had the right to "dispose" of a territory such as Puerto Rico, could it also strip Puerto Ricans of their citizenship? 

We argue that the citizenship rights enjoyed by Puerto Ricans today - granted by statute, and solidified by nearly a century of historical practice - are not compatible with an unrestrained Congressional power to expel the island, and that the latter must yield. That, in turn, suggests that perhaps Puerto Rico is no longer "foreign" to the United States in any sense.

Joseph Blocher & G. Mitu Gulati, What Does Puerto Rican Citizenship Mean for Puerto Rico’s Legal Status?, 67 Duke Law Journal Online, 122–132 (2018).