Any federal system needs a strategy for dealing with potential conflicts between the national and local governments. There are at least three strategies available. First, each government could be given exclusive jurisdiction over its respective sphere, which would avoid altogether the possibility of direct conflict. Second, the governments could have concurrent jurisdiction, but one government could be given power to veto actions of the other, either in the event of actual conflict or in general classes of cases. Third, both governments could be allowed to act without mutual interference, but one government’s acts could be given primacy over the other’s acts in the event of actual conflict.

The Supremacy Clause embodies the third strategy. It is a conflict-of-laws rule specifying that certain national acts take priority over any state act that conflicts with national law. In this respect, the Supremacy Clause follows the lead of Article XIII of the Articles of Confederation, which provided that “[e]very state shall abide by the determinations of the united states in congress assembled, on all questions which by this confederation are submitted to them.” While the last portion of the Supremacy Clause specifically singles out only state court judges as bound by federal law, the declaration of federal supremacy in the clause’s opening portion is general and binds all legal actors, ranging from federal officials to state jurors. In fact, Article VI, Clause 3, the Oaths Clause, requires all legislative, executive, and judicial officers of both state and federal governments to swear to support the U.S. Constitution.

The Supremacy Clause does not distinguish among the three named sources of federal law: the Constitution, the laws of the United States, and treaties. All are equally supreme over competing sources of state law or other sources of federal law (such as federal common law). Thus, the Supremacy Clause does not itself establish the supremacy of the Constitution over federal statutes or treaties. Rather, constitutional primacy over other sources of “supreme” federal law is a structural inference from the nature of the Constitution—as elaborated by Chief Justice John Marshall in Marbury v Madison (1803). The sequencing of sources of federal law in the Supremacy Clause, with the Constitution coming first, is some modest evidence in favor of constitutional primacy, which is precisely how Marshall’s argument in Marbury employed the clause. One could use similar intratextual and structural considerations to argue that federal statutes must always take precedence over federal treaties, but standard law has long been that federal statutes and treaties are equally supreme, with the latest enactment controlling in the event of conflict between them.

Modern law also treats federal administrative regulations as supreme over competing sources of state law. See Geier v. American Honda Motor Co., Inc. (2000). This conclusion rests more uneasily with the language of the Supremacy Clause, which names only the Constitution, the laws of the United States made pursuant thereto, and treaties as supreme federal instruments. Normally, one thinks of the “Laws of the United States . . . made in Pursuance” of the Constitution as statutes enacted in accordance with the lawmaking procedures of Article I, Section 7, and administrative regulations do not fit that description. But the courts have attempted to resolve the possible contradiction by holding that administrative regulations are made pursuant to a delegation of law from Congress. Thus, they take on the character of federal “laws.” Whatever the correct answer may be as a matter of original meaning, the principle of federal regulatory supremacy over state law is now firmly established. Indeed, federal regulations have emerged as the most frequent source of federal-state conflicts.

The Supremacy Clause does not grant power to any federal actor, such as Congress. It is an interpretative rule that deals with resolving conflicts between the federal and state governments once federal power has been validly exercised. It does not preclude other strategies for dealing with potential national and state conflicts, nor does it allocate power between the national and state governments. Other parts of the Constitution do that.

There was support at the Constitutional Convention for a supremacy clause that would adopt other conflict-resolving strategies. James Madison, among others, favored a direct congressional power to veto state laws, and he even seconded the strong proposal of Charles Pinckney “that the National Legislature shd. have authority to negative all [state] Laws which they shd. judge to be improper.” The Convention repeatedly rejected all such proposals for a federal veto power over state laws. The objective of the Framers throughout was to devise strategies that would reduce occasions for national and state conflict.

The Supremacy Clause in its final form was adopted by the Convention without serious dissent. Indeed, the essence of its final form was proposed by the Anti-Federalist Luther Martin. While some Anti-Federalists subsequently objected in broad terms to the prospect of federal supremacy, nothing in those debates negated the general understanding that the Supremacy Clause was a straightforward conflict-of-laws rule designed to resolve conflicts between state and federal law touching on the same subject.

The clause’s language, context, and history leave some important questions unanswered. For example, what constitutes a conflict? Must it be literally impossible to comply with both the state and federal rules, or is it enough that a state’s law will in some fashion alter or stand as an obstacle to the operation of the federal rule?

Properly applied as a conflict-of-laws provision, the Supremacy Clause would lead a common law court to acknowledge that a conflict does not always occur simply because two sovereigns have legislated on a common subject; both Congress and the courts recognize that principle today.

Consequently, the modern Court has fashioned subsidiary rules to try to determine when there is a genuine conflict between a state and federal law on the same subject, or, in modern parlance, whether the federal law has “preempted” the state law. Modern doctrine generally holds that preemption occurs whenever it is intended by Congress. That intent, of course, can most directly be demonstrated by an express provision in a federal statute declaring the statute’s preemptive effect (or lack thereof). Even in the absence of an express preemption provision, however, state law is preempted “[w]hen Congress intends federal law to ‘occupy the field’” or “to the extent of any conflict with a federal statute.” Crosby v. National Foreign Trade Council (2000). Conflicts can also result either when it is literally impossible to comply with both state and federal law, Pliva, Inc. v. Mensing (2011), or, much more commonly, when a state law “stands as an obstacle to the accomplishment and execution of the full purposes and objectives of Congress.” Hines v. Davidowitz (1941). Determining whether a state law sufficiently obstructs federal purposes and is thus preempted “is a matter of judgment, to be informed by examining the federal statute as a whole and identifying its purpose and intended effects.” Crosby. There is, however, an interpretative presumption against preemption in areas of traditional state concern. As the Court stated in Rice v. Santa Fe Elevator Corp. (1947), “[W]e start with the assumption that the historic police powers of the States were not to be superseded by the Federal Act unless that was the clear and manifest purpose of Congress.”

The preemption doctrine in its current form is a twentieth-century development. No state statute was invalidated for anything other than a straightforward conflict with a specific federal enactment until 1912, and the focus on congressional intent as the touchstone of preemption did not emerge until the New Deal, when the locus of reformist legislation shifted from the states to the federal government.

In addition to serving a central role in preemption analysis, the Supremacy Clause is often seen as the source of the principle that states cannot regulate, interfere with, or control dera. . . instrumentalities. This principle is generally traced to McCulloch v. Maryland (1819), in which the Court held that Maryland could not constitutionally tax the operations of the Bank of the United States. Chief Justice Marshall declared in McCulloch that

[i]t is of the very essence of supremacy to remove all obstacles to its action within its own sphere, and so to modify every power vested in subordinate governments, as to exempt its own operations from their own influence. This effect need not be stated in terms. It is so involved in the declaration of supremacy, so necessarily implied in it, that the expression of it could not make it more certain.

Modern law has to some extent qualified the broadest implications of this early formulation of the supremacy principle. If federal supremacy indeed “remove[s] all obstacles” to federal action that might be posed by state regulation, states could be constitutionally forbidden even from taxing the salaries of federal employees. The Court indeed embraced such an idea for some time before specifically rejecting it in Graves v. New York ex rel. O’Keefe (1939). Modern law maintains instead that “[a] state regulation is invalid only if it regulates the United States directly or discriminates against the Federal Government or those with whom it deals.” North Dakota v. United States (1990) (plurality opinion).

While the federal government can prevent states from interfering with federal operations, whether through taxes or otherwise, that does not necessarily mean that the Supremacy Clause is the basis upon which Congress exercises its power to protect federal operations, for the Supremacy Clause is not a grant of power to Congress. Rather, the valid exercise of any one of Congress’s enumerated powers can constitute the constitutional source of a statute that effectively preempts a state law. In particular, the Necessary and Proper Clause would be a vehicle for a statute that explicitly disables state law from operating in an area of federal concern. Thus, for an explicitly preemptive statute to be constitutional, it must be “necessary and proper for carrying into execution” some enumerated federal power, subject, of course, to the constitutional limits of the Necessary and Proper Clause itself.

For example, Congress could decide (explicitly or implicitly) that it alone should regulate the radiological-safety aspects involved in the construction and operation of a nuclear plant and thus preempt the field from any state regulation of nuclear power safety. Pacific Gas & Electric v. Energy Resources Commission (1983). Congress could decide (explicitly or implicitly) that it wanted gradually to phase in passive restraints in automobiles, thus preempting a local tort law that required airbags to be installed in all new cars. Geier v. American Honda Motor Co., Inc. Congress might decide that it wanted an area in interstate commerce to be regulated only by the free market and not by the states, thus precluding state legislation in this particular area altogether.

Inasmuch as any state statute that regulates federal activities in ways forbidden by a congressional statute would conflict with valid federal law, Congress is thus logically free to permit state regulation of federal instrumentalities through a sufficient expression of intent. For example, the Supreme Court has allowed Congress either to authorize or to limit state taxation of federal banks. Carson v. Roane-Anderson Co. (1952). In any event, the sequence is this: Congress, under its delegated powers, or a state, under its police power, may establish legal rules dealing with the same subject. It then falls to the courts to determine, under the Supremacy Clause, whether the state and federal rules are in conflict.

Article VI, Section 2, has separate provisions for treaties and federal laws. There is a textual distinction in the clause between laws “made in pursuance [of the Constitution]” and treaties “made under the authority of the United States.” See Missouri v. Holland (1920). The effectiveness of national treaties was a special concern of the Founding generation. This language ensured that treaties entered into by the United States prior to ratification of the Constitution—most notably, the 1783 treaty of peace with Great Britain and its guarantees against confiscations of loyalist property—took precedence over conflicting state laws, and the language in the Supremacy Clause targeting state court judges no doubt reflected the concern about treaty enforcement. The phrasing does not in any way imply that treaties are “supreme” even if they conflict with other constitutional provisions. The Supreme Court has declared that neither a treaty approved by the Senate nor an executive agreement made under the president’s authority can create obligations that violate constitutional guarantees such as found in the Bill of Rights. Reid v. Covert (1957).

Like federal statutes, treaties are “supreme” only when they are effective as domestic law. Thus, the manner in which treaties become legally effective is important for determining when they take priority over state law. “Self-executing treaties” become part of the law of the United States directly. On the other hand, the courts will not enforce “non-self-executing treaties” until they are carried into law by an act of Congress. Determining whether a treaty is self-executing or non-self-executing is a complex and confusing task, as lower courts have readily averred. In general, the courts will regard a treaty as non-self-executing if it requires any governmental funding to accomplish its purposes, or if there is any expressed intent by the terms of the treaty, the president, the Senate, or even the record of negotiation that indicates that the government desired that the treaty be non-self-executing.

In addition, there is a vigorous debate among scholars over what was the Framers’ original understanding on this point. One group holds that the Framers intended that most treaties were to be self-executing (unless the terms of the treaty indicate otherwise). Another group of commentators argues that any treaty that impinges upon Congress’s Article I powers is non-self-executing. Otherwise, the Framers’ careful system of protecting the people from onerous legislation through the separation of powers could be out-flanked by the president and the Senate alone.

However a treaty becomes part of the law of the United States, it is on a par with other federal laws and can be repealed by Congress, though the United States’ obligations under international law remain. Under Supreme Court precedents, the last expression of the sovereign will controls what will be enforced, so an act of Congress that is in conflict with a treaty will control if the act became law after the Senate ratified the treaty, and vice versa. To avoid such conflicts, the courts have fashioned a prudential rule whereby laws will be interpreted to be in harmony with United States treaty obligations if at all possible.