"The Constitution, and the laws of the United States which shall be made in pursuance thereof... shall be the supreme law of the land..."
But when the Federal government fails to follow the Constitution and protect the states, it's up to the states to take action. The Federal government does have to defend the states as stated in Article IV, Sec. 4:
"The United States shall guarantee to every state in the union a republican form of government, and shall protect each of them against invasion; and on application of the legislature, or of the executive (when the legislature cannot be convened) against domestic violence."
Arizona has asked the Feds continually for military assistance and the Feds would simply ignore their cry for help. Arizona and many other states along the border are being invaded by illegal immigrants. In fact, the Feds just gave an 80 mile strip of Arizona to Mexico by putting up signs around it saying not to enter because it's dangerous do to the drug war.
Because of Arizona's action, many other states are planning on making legislation very much like Arizona's.
Our Founding Fathers wrote the Constitution believing that the We the People would stay awake and keep the Feds in check. But we failed on that part and now we have a Progressive Congress and White House. But we can change much of that with this year's votes. We must protect our states from enemies both foreign and domestic.