The 10th Amendment to the US Constitution was ratified as part of the Bill of Rights on December 15, 1791. It is the key amendment reinforcing the idea of federalism. In other words, that unless the Constitution specifically gave a power to the federal government or denied it to the states, then it was reserved for the states. Over time, the federal government has taken more power from the states as the courts have given it wider powers due to amendments such as the 14th amendment. Text of the 10th Amendment 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.