The United States has a constitution-based federal system. Federalism is a form of government in which power is divided between a national (federal) government and local (state) governments.
The Constitution's Supremacy Clause states that federal laws are the "supreme law of the land," after only the Constitution itself. However, it is clear that the Constitution limits the federal government's powers. Those powers the federal government does have are listed, or enumerated, by the Constitution. Every law made by Congress must be based on one or more of these enumerated powers.
The Federal Government Maintains the Military
The federal government's "enumerated powers" are listed in Article I, Section 8 of the Constitution. Among other things, they include: the power to levy taxes, regulate commerce, create federal courts (underneath the Supreme Court), set up and maintain a military, and declare war.
In addition, the Constitution's Necessary and Proper Clause lays out certain "implied powers." These are needed to carry out the enumerated powers. As Supreme Court Justice John Marshall noted in 1819, a government entrusted with great powers must also be entrusted with the power to execute them.
What the States Decide
The Constitution thus grants broad powers to the federal government. However, these powers are limited by the 10th Amendment. That amendment declares that those powers not specifically reserved for the federal government are given to the states.
As Founding Father James Madison explained, the powers given to each state "concern the lives, liberties, and properties of the people." They serve to advance and protect "the internal order, improvement, and prosperity of the State."
State powers are those required for public safety, health, and welfare. They are generally called "police powers."
State Laws Must Give Way to Federal Laws
Finally, certain powers are called "concurrent powers." These are powers that states and the federal government both may exercise concurrently, or at the same time. They include the power to set up courts, to levy taxes, and to spend and borrow money.
In certain areas of life, it can be difficult to determine whether the federal government or a state government has the power to make laws. This can lead to two conflicting sets of laws.
The problem of conflicting state and federal laws has given rise to what is called the doctrine of preemption. Under this doctrine, if a state or local law conflicts with a federal law, the state or local law must give way. The Constitution denies the states the power to block, delay, or alter federal laws enacted by Congress.
An Exception Is an Unconstitutional Law
There are a variety of situations in which federal law preempts, or overrides, state law. Preemption occurs when there is an explicit, or clear and obvious, conflict between federal and state laws. It also occurs when it would be impossible for someone to follow both state and federal laws. Finally, it occurs when the state law would prevent the realization of the purposes and objectives of the federal law.
The only exception to the doctrine of preemption is when a federal law is itself unconstitutional. In other words, a federal law is invalid if it exceeds the power of the federal government.