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.
James Madison reiterated this ideology in The Federalist Papers:
The powers delegated to the federal government are few and defined. Those which are to remain in the state governments are numerous and indefinite. The former will be exercised principally on external objects, [such] as war, peace, negotiation, and foreign commerce. The powers reserved to the several states will extend to all the objects which, in the ordinary course of affairs, concern the lives, liberties, and properties of the people.