Federal v. State
The American Constitution creates a federal system, dividing power between the federal government and the states. This system allows local custom to rule in most localities without sacrificing the overall government’s power to set certain plenary rules. The federal constitution limits the federal government by granting the federal government only those powers enumerated in the Constitution, for instance, questions involving interstate commerce. Powers not enumerated for the federal government in the federal constitution are left to the states (an idea made explicit by the Tenth Amendment to the U.S. Constitution). Any dispute about whether substantive state or federal law governs is always resolved in favor of the federal, per the U.S. Constitution’s Supremacy Clause.
Thus, there exists a two-tier system, where both the state and the federal levels have courts that exercise their own substantive laws. In this case, the U.S. Congress has created substantive criminal laws, and the United States has brought criminal charges in federal court. Conceivably, were Montana to have analogous crimes to those in the indictments in this case, the state could bring its own, separate criminal actions in state court.