Whether a tribe's authority to prosecute its own members for violations of tribal law comes from retained tribal sovereignty or from a delegation of federal power.
Holding
The authority is an exercise of the tribe's retained inherent sovereignty, not delegated federal authority.
Reasoning
The dual-sovereignty inquiry turns on the ultimate source of each prosecuting authority's power, not on how extensively one government may regulate or control the other. States and the Federal Government derive their authority from separate sources; in contrast, municipalities and federal territories exercise power that originates in the state or federal sovereign that created them. For that reason, successive prosecutions by a city and a state, or by a territory and the United States, are prosecutions by one sovereign rather than two.
Indian tribes were self-governing political communities before their incorporation into the United States. Although their sovereignty is now limited, tribes retain those sovereign powers that Congress has not withdrawn by treaty or statute, or that were not necessarily lost because of their dependent status. Internal self-government, including the power to define and punish offenses committed by tribal members, is among the powers generally retained.
The Navajo Tribe had not surrendered, and Congress had not eliminated, its authority to punish members for violations of tribal law. Treaties and federal statutes recognized tribal jurisdiction over internal tribal matters, while the General Crimes Act expressly preserved tribal authority in several circumstances. Nothing in the relevant law affirmatively granted the Navajo Tribe its basic power of criminal self-government, which further showed that the power was retained rather than federally delegated.
Federal regulation of tribal institutions did not alter the source of tribal power. Congress may regulate tribal government, require approval of tribal codes, apply statutory civil-rights protections, and limit tribal punishments. But the power to regulate how a tribe exercises self-government is different from creating that power. Talton v. Mayes likewise established that a tribe's prosecution of a tribal member under tribal law is an exercise of tribal, not federal, authority.