The Tenth Amendment to the U.S. Constitution reads:
The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States, respectively, or to the people.
Ratified in 1791, the Tenth Amendment to the Constitution embodies the general principles of Federalism in a republican form of government. The Constitution specifies the parameters of authority that may be exercised by the three branches of the federal government: executive, legislative, and judicial. The Tenth Amendment reserves to the states all powers that are not granted to the federal government by the Constitution, except for those powers that states are constitutionally forbidden from exercising.
For example, nowhere in the federal Constitution is Congress given authority to regulate local matters concerning the health, safety, and morality of state residents. Known as police powers, such authority is reserved to the states under the Tenth Amendment. Conversely, no state may enter into a treaty with a foreign government because such agreements are prohibited by the plain language of Article I to the Constitution.