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.
Read the 10th Amendment carefully. What does it do, other than stating the obvious? When you pull apart its somewhat awkward construction, it says simply that the powers not given to the national government remain with the states or the people unless the Constitution has explicitly forbidden that power to the states (for instance, the states are not allowed to print or coin their own money). It’s understandable, then, that the Supreme Court has called the 10th Amendment, “but a truism that all is retained which has not been surrendered.”
During the ratification debates, the Antifederalists had demanded that something like the 10th Amendment be added to the Constitution in order to prevent possible misinterpretations of the relationship between federal and state powers. Simply stated, the 10th Amendment was meant to affirm that the federal government could exercise no powers except those which were enumerated in the Constitution. All remaining powers were reserved to the state governments and the people. It is therefore what lawyers commonly call “a rule of construction,”—a guide to interpreting a law—in this case, a guide to interpreting the Constitution itself.
Despite the 10th Amendment’s history and (relatively) plain language, the Supreme Court has, at various times in our nation’s history, found that the amendment is more than “a truism”—that it does more than simply describe the difference between state and national power. In the late 19th and early 20th centuries, the Court often interpreted the 10th Amendment as limiting the powers given to the national government. The Court reversed course at the end of the Great Depression, and rarely cited the 10th Amendment for the following 50 years. But then, in the 1990s, the Court twice cited the 10th Amendment as at least part of its rationale for striking down intrusive federal statutes.