The U.S. Supreme Court's split decision on Arizona's draconian immigration law leaves little doubt about one fundamentally crucial matter: immigration enforcement primarily is a federal responsibility.

Politicians are free to argue as to the effectiveness or priorities of the federal government's policy, but they are not free to become freelance substitutes for the appropriate federal authorities.

The court upheld a part of the Arizona law allowing state or local police to determine the immigration status of people they stop or arrest for something else

But the justices struck down the ability of local authorities to do much beyond that. They struck down provisions that had allowed the arrests without warrant of someone police suspect of being deportable, another section making it a state crime for someone without proper documents to seek work, and another making it a state crime for immigrants to fail to register with the federal government.

And even while sustaining the section allowing police to determine a detainee's status, the court left open the door for further challenges of that provision.

The most important principle arising from the multiple aspects of the decision is the most fundamental one, as stated in an opinion by Justice Anthony Kennedy: "Arizona may have understandable frustrations with the problems caused by illegal immigration while that process continues, but the state may not pursue policies that undermine federal law."