The Supremacy Clause is that which derives from Constitutional law and sets forth that three distinct areas of legislation be at the forefront. It states that the Constitution, Federal statutes, and United States treaties encompass the "supreme law of the land", therefore making them the highest areas of law possible within the legal system of America. The Supremacy Clause may be found in Article VI, Section 2 of the United States Constitution.
A landmark case representing one of the earliest examples of the use of the Supremacy Clause is that of McCulloch v. Maryland. In this case, the Supreme Court ruled that the State of Maryland had no legal right to tax the Second Bank of the United States as a Federal entity. This exhibited how the Supremacy Clause called into question the actions of the State, and therefore, made it so that the State could not legally tax the Federal Government.
Another case that made use of the Supremacy Clause in connection with Constitutional law was that of Missouri v. Holland. This Supreme Court case was conducted over the cause of international treaties. The Court ruled that the power of the Federal Government to enforce treaties overrode that of the State's authority to voice concerns as to the violation of their local rights as prescribed from the 10th Amendment.
This Amendment was used by the Supreme Court following the Civil War and stated that states assumed the rights to powers not already set forth for the Federal Government. This did not last long, however, as everything was shifted to the Government to have vast national power, which meant that the Federal Government could not be subject to State law aside from by its own volition.
In addition, the Supremacy Clause also maintains that State legislatures assume, in one way or another, the guidelines and procedures set forth by the Federal Government. This is due to the presentation of two issues that stem from State and Federal conflict. These include Congress' surpassing of its original authority as well as its overall intent in going over that of State policy. In both cases, Congress may be acting with the express authority of creating uniformity of legislature. In such a way it may be attempting to enable the coexistence of Federal and State government.
A case that highlighted such issues of Federal law presuming power over State action is that of Pennsylvania v. Nelson. In this case, the Supreme Court instituted qualifications for when the Government does encroach upon the rule of states, even when absent of apparent intent. These include that the Federal law is so extensive that states may not be able to adequately supplement it, the fact of the "Federal interest's dominance," and whether "State law" is in so much of a contrast to the Federal administration that it may only do harm to it. Such cases represent the ways in which the Supremacy Clause has been employed.