dec 15, 1791 - 10th Amendment
Description:
The last amendment of The Bill of Rights states that "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." In other words, all powers not explicitly stated in the constitution belong to each of the States or to the citizens.
This decreases the federal government's power tremendously by preventing it to stretch beyond what is explicitly stated in the constitution and leaving those powers to the state and local governments.
Added to timeline:
Date: