Here is the opinion in Commonwealth of Puerto Rico v. Sanchez Valle.

P.R. had tried to analogize itself to Indian nations, but that effort failed:

For similar reasons, Indian tribes also count as separate sovereigns under the Double Jeopardy Clause. Originally, this Court has noted, “the tribes were self-governing sovereign political communities,” possessing (among other capacities) the “inherent power to prescribe laws for their members and to punish infractions of those laws.” Wheeler, 435 U. S., at 322–323. After the formation of the United States, the tribes became “domestic dependent nations,” subject to plenary control by Congress—so hardly “sovereign” in one common sense. United States v. Lara, 541 U. S. 193, 204 (2004) (quoting Cherokee Nation v. Georgia, 5 Pet. 1, 17 (1831)); see Santa Clara Pueblo v. Martinez, 436 U. S. 49, 56 (1978) (“Congress has plenary authority to limit, modify or eliminate the [tribes’] powers of local self-government”). But unless and until Congress withdraws a tribal power—including the power to prosecute—the Indian community retains that authority in its earliest form. See Wheeler, 435 U. S., at 323. The “ultimate source” of a tribe’s “power to punish tribal offenders” thus lies in its “primeval” or, at any rate, “pre-existing” sovereignty: A tribal prosecution, like a State’s, is “attributable in no way to any delegation . . . of federal authority.” Id., at 320, 322, 328; Santa Clara Pueblo, 436 U. S., at 56. And that alone is what matters for the double jeopardy inquiry. 

Slip. op. at 9-10. So now we can say tribal sovereignty is “primeval”: whether we should is another question.

The dissent, authored by Justice Breyer and joined by Justice Sotomayor, argues that tribal sovereignty actually does derive from Congress, by virtue of Congress not divesting it completely:

But as the Court today recognizes, this prelapsarian independence must be read in light of congressional action—or, as it were, inaction. That is because—whatever a tribe’s history— Congress maintains “plenary authority to limit, modify or eliminate the [tribes’] powers of local self-government,” Santa Clara Pueblo v. Martinez, 436 U. S. 49, 56 (1978), and thus the tribes remain sovereign for purposes of the Double Jeopardy Clause only “until” Congress chooses to withdraw that power, ante, at 10. In this sense, Congress’ pattern of inaction (i.e., its choice to refrain from withdrawing dual sovereignty) amounts to an implicit decision to grant such sovereignty to the tribes. Is not Congress then, in this way, the “source” of the Indian tribes’ criminal enforcement power?

Dissent at 5. No no no!!!!

Fortunately, the Court rejects this formulation, calling it “deeply disturbing”:

This Court’s reasoning could not have been plainer: The States (all of them) are separate sovereigns for double jeopardy purposes not (as the dissent claims) because they exercise authority over criminal law, but instead because that power derives from a source independent of the Federal Government. See Heath, 474 U. S., at 89. So too for the tribes, see supra, at 9–10; and, indeed, here the dissent’s contrary reasoning is deeply disturbing. According to the dissent, Congress is in fact “the ‘source’ of the Indian tribes’ criminal enforcement power” because it has elected not to disturb the exercise of that authority. Post, at 5. But beginning with Chief Justice Marshall and continuing for nearly two centuries, this Court has held firm and fast to the view that Congress’s power over Indian affairs does nothing to gainsay the profound importance of the tribes’ pre-existing sovereignty. See Worcester v. Georgia, 6 Pet. 515, 559–561 (1832); Talton v. Mayes, 163 U. S. 376, 384 (1896); Michigan v. Bay Mills Indian Community, 572 U. S. ___, ___–___ (2014) (slip op., at 4–5). And once again, we have stated in no uncertain terms that the tribes are separate sovereigns precisely because of that inherent authority.

Slip. op. at 11 n. 5. Whew!