The Supremacy Clause states that the Constitution, the laws of the United States and all treaties under the authority of the United States are deemed the supreme law of the land, meaning it overrides state constitutions and laws. It is the second clause of Article VI of the Constitution.
According to Cornell University Law School, McCulloch v. Maryland exemplifies how federal law prevails over state authority. In this landmark case, the state of Maryland taxed banks that did not have a state charter. When the U.S. Congress incorporated a federal bank in Maryland, a cashier named McCulloch employed by the bank received a judgment from the state for not collecting a state tax. The Supreme Court reversed the judgment, holding that the state did not have authority to tax the federal government.