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.
The second 'catch-all' provided by our Founding Fathers. The ninth provided for the natural rights we are all 'endowed with by our Creator' Life, liberty and the pursuit of happiness. The tenth may very well be the most important of all previous rights enumerated.
The tenth amendment makes it clear that the United States' power comes from the consent of the governed. These powers enumerated limit the government and empower the people. Any other rights that our citizens have or may not have, if they are not prohibited by the States these rights are left to the States or the people.
While Federal law today trumps State law, when it comes to making decisions in government, when the Federal government is failing to do the job, it comes down to States Rights. In other words, let the states decide how to govern their laws and people.
There was a deep disagreement amongst early framers of our country that argued that the tenth amendment should be the last amendment to the Constitution. Don't get me wrong, some of the remaining amendments that have been added were for a good cause. Women's voting rights, etc. The public schools taught me that the Civil War was fought over slavery. That is simply not true.
The Civil War was fought over states rights. Liberals have a hard time explaining why freed black men were fighting on the side of the Confederacy if the Civil War was only about slavery. The states, Constitutionally, have the right to decide for themselves and when the Federal Government comes in and institutes laws that the people of the state oppose, it leaves the citizens of that state feeling disenfranchised.
The Tenth Amendment grants the power to the States and to the People. How about an illustration? Obama has made it clear that he wants to 'fundamentally transform' this country. The federal government decides to make preaching against homosexuality a hate crime. He has already shown that he opposes the Constitution so don't think this can't happen. The states have a right to say 'no' to that law. Because the power is with the states and the people.
The federal government now decides that, like China, families are now limited to only two children. Any future pregnancies must by law be terminated by abortion. The states and the people have the right to disobey that law, because the power is with the states and the people.
The federal government now requires a two year service in Obama's Civilian army in order to keep citizenship in America. Failure to report for duty, failure to send your teenagers in to complete their service will result in a loss of citizenship status and end in jail time. The states and the people have the right to disobey that law, because the power is with the states and the people.
Simply put, Constitutionally speaking, the Federal Government has no power to create, change or deny rights to the citizens. States have rights for the express purpose of keeping power hungry Marxists like our President from seizing total control over the country and fundamentally transforming it into a Socialist-Fascist regime.