Now that the U.S. Supreme Court has upheld the most controversial piece of Arizona’s immigration law — while throwing out much of the rest of the legislation — Michael Barone uses his latest column to probe the next steps in the immigration debate.

Barack Obama and his administration have taken heart that the court overturned Arizona’s state penalties for illegal immigrants. The idea is that states can’t pile higher penalties on top of those voted by Congress, just as states can’t deport people whom Congress allowed into the country.

But the much more significant part of the case was the unanimous 8–0 (Justice Kagan not voting) ruling upholding the Arizona provision authorizing state and local law-enforcement personnel to help enforce federal law by asking those stopped for other reasons to show that they are citizens or legal immigrants.

This has been derided as a “where are your papers?” provision redolent of an authoritarian regime. But federal law has long required legal immigrants to carry their papers. And just about every adult carries a driver’s license or equivalent without feeling oppressed.

What seems out of date now is the attitude, common in some liberal circles, that it’s unsporting, if not oppressive, to enforce the law against illegal immigrants. Cities such as San Francisco have declared themselves “sanctuary cities,” with no obligation to enforce federal laws they don’t like.

The underlying theory seems to be that it’s unjust to bar anyone from entering our country. But that’s obviously nonsense. Under international law, we have no obligation to admit anyone to the United States except accredited diplomats. We open our borders to visitors and legal immigrants not because we have to, but because we think it’s in our interest to do so.