The U.S. Constitution establishes a system of dual sovereignty between the states and the federal government, with each state having its own government, endowed with all the functions essential to separate and independent existence. Although the Supremacy Clause of the Constitution designates "the Laws of the United States" as "the supreme Law of the Land," other provisions of the Constitution-as well as legal principles undergirding those provisions-nonetheless prohibit the national government from enacting certain types of laws that impinge upon state sovereignty. The various principles that delineate the proper boundaries between the powers of the federal and state governments are collectively known as "federalism." Federalism-based restrictions that the Constitution imposes on the national government's ability to enact legislation may inform Congress's work in any number of areas of law in which the states and the federal government dually operate. There are two central ways in which the Constitution imposes federalism-based limitations on Congress's powers. First, Congress's powers are restricted by and to the terms of express grants of power in the Constitution, which thereby establish internal constraints on the federal government's authority. The Constitution explicitly grants Congress a limited set of carefully defined enumerated powers, while reserving most other legislative powers to the states. As a result, Congress may not enact any legislation that exceeds the scope of its limited enumerated powers. That said, Congress's enumerated powers nevertheless do authorize the federal government to enact legislation that may significantly influence the scope of power exercised by the states. For instance, subject to certain restrictions, Congress may utilize its taxing and spending powers to encourage states to undertake certain types of actions that Congress might otherwise lack the constitutional authority to undertake on its own. Similarly, the Supreme Court has interpreted the Constitution's Commerce Clause to afford Congress substantial (but not unlimited) authority to regulate certain purely intrastate economic activities that substantially affect interstate commerce in the aggregate. Congress may also enact certain types of legislation in order to implement international treaties. Additionally, pursuant to a collection of constitutional amendments ratified shortly after the Civil War, Congress may directly regulate the states in limited respects in order to prevent states from depriving persons of certain procedural and substantive rights. Finally, the Necessary and Proper Clause augments Congress's enumerated powers by empowering the federal government to enact laws that are "necessary and proper" to execute its express powers. In addition to the internal constraints on Congress's authority, the Constitution also imposes external limitations on Congress's powers vis-à-vis the states-that is, affirmative prohibitions on certain types of federal actions found elsewhere in the text or structure of the Constitution. The Supreme Court has recognized, for instance, that the national government may not commandeer the states' authority for its own purposes by forcing a state's legislature or executive to implement federal commands. Nor may Congress apply undue pressure to coerce states into taking actions they are otherwise disinclined to take. Furthermore, the principle of state sovereign immunity-which limits the circumstances in which a state may be forced to defend itself against a lawsuit against its will-imposes significant constraints on Congress's ability to subject states to suit. Finally, the Supreme Court has recognized limits to the extent to which Congress may subject some states to more onerous regulatory burdens than other states.