The Supremacy Clause, found in Article VI of the U.S. Constitution, establishes the Constitution, Federal Statutes, and U.S. treaties as "the supreme law of the land." Therefore, if a state law conflicts with a federal law, the federal law must be followed.
The Supremacy Clause states:
"This Constitution, and the laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the authority of the United States, shall be Supreme Law of the land; and the Judges in every state shall be bound thereby, any thing in the Constitution or Laws of any state to the contrary notwithstanding."
According to U.S. law treaties are those international agreements that receive the advice and consent of the Senate. (Article II, section 2, clause 2 of the Constitution). A treaty to which United States is a party is given status equal to that of a federal legislation and therefore forms a part of the Supreme law of the land.
This concept of federal supremacy was first developed by Chief Justice John Marshall in McCulloch v. Md., 17 U.S. 316, 406 (U.S. 1819), where the court held that the State of Maryland could not tax the Second Bank of United States, a branch of the National Bank. It was concluded that "the government of the Union, though limited in its power, is supreme and its laws, when made in pursuance of the constitution, form the supreme law of the land, "any thing in the constitution or laws of any State to the contrary notwithstanding."
In Edgar v. Mite Corp., 457 U.S. 624, 632 (U.S. 1982) it was held that “a state statute is void to the extent that it actually conflicts with a valid federal statute” and that a conflict will be found either where compliance with both federal and state law is impossible or where the state law stands as an obstacle to the accomplishment and execution of the full purposes and objectives of Congress.
Similarly in Stone v. San Francisco, 968 F.2d 850, 862 (9th Cir. Cal. 1992) the court held on the issue of injunction and remediation, that "otherwise valid state laws or court orders cannot stand in the way of a federal court's remedial scheme if the action is essential to enforce the scheme. State policy must give way when it operates to hinder vindication of federal constitutional guarantees."