Supreme Court Decision. Downes v. Bidwell, (one of the Insular Cases) 1901.

We are also of opinion that the power to acquire territory by treaty implies, not only the power to govern such territory, but to prescribe upon what terms the United States will receive its inhabitants, and what their status shall be in what Chief Justice Marshall termed the "American empire.". . . Indeed, it is doubtful if Congress would ever assent to the annexation of territory upon the condition that its inhabitants, however foreign they may be to our habits, traditions, and modes of life, shall become at once citizens of the United States. In all its treaties hitherto the treaty-making power has made special provisions for this subject. . . . In all these cases there is an implied denial of the right of the inhabitants to American citizenship until Congress by further action shall signify its assent thereto. . . .

It is obvious that in the annexation of outlying and distant possessions grave questions will arise from differences of race, habits, laws and customs of the people, and from differences of soil, climate and production, which may require action on the part of Congress that would be quite unnecessary in the annexation of contiguous territory, inhabited only by people of the same race, or by scattered bodies of native Indians.