The U.S. is at war -- with itself. The federal guys are suing the state gal (AZ) over enforcement of immigration laws. The clash comes down to this: the state of Arizona wants to enforce federal laws that the US federal government has no intention of enforcing.
The following is satire:
Arizona: "But, we have no actual intention of enforcing this law, either."
The US: "Doesn't matter. We assert the sole right to not enforce this law. If we have states piling on willy-nilly not enforcing federal laws, where will it all end?!"
Where will it end?, indeed.