When the Obama administration decided that it had no interest in preventing the movement of undocumented aliens from Mexico into the southwest United States, the State of Arizona decided to take matters into its own hands. Based on a novel theory of constitutional law, namely, that if a state is unhappy with the manner in which federal law is being enforced or not being enforced, it can step into the shoes of the feds and enforce federal law as it wishes the feds would, it enacted legislation to accomplish that.
The legislation created two conflicts that rose to the national stage. The first is whether any government may morally and legally interfere with freedom of association based on the birthplace of the person with whom one chooses to associate. The second is whether the states can enforce federal law in a manner different from that of the feds.