"Federalism" is the process by which two or more governments share powers over the same geographic area.
In the United States, the Constitution grants certain powers to both the U.S. government and the state governments.
For example, under Article I, Section 8 of the Constitution, grants the U.S. Congress certain powers such as coining money, regulating interstate trade and commerce, declaring war, raising an army and navy and to establish laws of immigration.
Things the states cannot do are listed in Article I, Section 9. Among these, states are forbidden from coining money, entering into treaties, charging duties on imports and exports and declaring war.
Under the 10th Amendment, powers not specifically listed in the Constitution, like requiring drivers licenses and collecting property taxes, are among the many powers "reserved" to the states.
“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.” – Tenth Amendment to the U.S. Constitution
The line between the powers of the U.S. government and those of the states is usually clear. Sometimes, it is not. Whenever a state government's exercise of power might be in conflict with the Constitution, we end up with a battle of “states' rights” which must often be settled by the Supreme Court.
Probably the greatest battle over states' rights -- segregation -- took place during the 1960's civil rights struggle.
Segregation: The Supreme Battle of State's Rights
In 1954, the Supreme Court in its landmark Brown v. Board of Education decision ruled that separate school facilities based on race are inherently unequal and thus in violation of the 14th Amendment which states, in part: "No state shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any state deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws."
However, several predominately Southern states, chose to ignore the Supreme Court’s decision and continued the practice of racial segregation in schools and other public facilities.
The states based their stance on the 1896 Supreme Court ruling in Plessy v. Ferguson. In this historic case, the Supreme Court, with only one dissenting vote, ruled racial segregation was not in violation of the 14th Amendment if the separate facilities were "substantially equal."
In June of 1963, Alabama Governor George Wallace stood in front of the doors of the University of Alabama preventing black students from entering and challenging the federal government to intervene. Later the same day, Wallace gave in to demands by Asst. Attorney Gen. Nicholas Katzenbach and the Alabama National Guard allowing black students Vivian Malone and Jimmy Hood to register.
During the rest of 1963, federal courts ordered the integration of black students into public schools throughout the South. In spite of the court orders, and with only 2 percent of Southern black children attending formerly all-white schools, the Civil Rights Act of 1964 authorizing the U.S. Justice Department to initiate school desegregation suits was signed into law by President Lyndon Johnson.
A less momentous, but perhaps more illustrative case of a constitutional battle of "states' rights" went before the Supreme Court in November 1999, when the Attorney General of the United States Reno took on the Attorney General of South Carolina Condon.