The federal judge who struck down most of Arizona’s new immigration law last week wasn’t trying to strike a blow for immigrants’ rights...She was making a much more fundamental argument, one that has regularly emerged in America’s long and often ugly history in dealing with noncitizens and other vulnerable minorities. The message was that Arizona cannot have its own immigration or foreign policy. It cannot tell the federal government how to enforce its laws. It is not up to any state to seize the power to upend federal priorities, particularly to wield a blunt enforcement tool that will do harm...