Following yesterday’s oral arguments, the early consensus among Supreme Court–watchers is that the conservative justices will band together to uphold at least some core provisions of Arizona’s controversial immigration law. One would hope that any such decision would be rooted in a strong understanding of the issues at stake. Unfortunately, after attending yesterday’s oral arguments, I’m not sure that the leader of the Court’s conservative bloc—Antonin Scalia—has a very sound grasp of our country’s immigration policies.

The question before the Court is whether S.B. 1070, the hard-line immigration enforcement law Arizona passed in 2010, complements or hinders federal immigration policy. The federal government argues that Arizona’s law goes far beyond federal immigration policy, unconstitutionally interfering in an area where Arizona lacks authority. Arizona argues that its law merely supplements federal efforts. To be sure, the Court faces a constitutional question, but one that is crucially rooted in a policy problem: Does Arizona’s law interfere with federal strategy?

Early on in his argument, Paul Clement—the former solicitor general and conservative legal superstar who is representing Arizona—faced a simple question from Justice Ginsburg: How can the state know if an undocumented immigrant is removable under immigration law? Sometimes, she pointed out, it can be complex. Ginsburg’s question was actually rather limited in scope—she was asking about the technical process of verifying immigration status with federal authorities—but it pointed to a core problem of immigration policy: It’s frequently hard to know exactly who ought to be deported.

Justice Scalia seemed not to appreciate this. During his questioning, he declared that Arizona’s efforts were hardly in tension with federal policy. “Arizona is not trying to kick out anyone who the federal government has not already said do not belong here,” he insisted. That’s not quite right. Arizona’s law mandates a maximum level of enforcement against all undocumented immigrants and creates a criminal penalty for being undocumented. By contrast, federal immigration policy has for years recognized that not all undocumented immigrants are the same. Some are a high priority, but some are not—and the administration’s policy officially recommends deferring enforcement action against “low-priority” immigrants, such as those who would benefit from the DREAM Act, the elderly, or people with close family ties to the United States.

One of the reasons that Washington’s approach to immigration—unlike Arizona’s—is so complicated is that federal policymakers need to take account of geopolitical considerations when deciding which undocumented immigrants to target for deportation. Donald Verrilli, Obama’s solicitor general, said as much, arguing that a maximal approach to immigration enforcement could create problems for U.S. relations with Mexico. “So we have to enforce our laws in a manner that will please Mexico?!” Scalia protested. But that’s a rather unfair way of describing Washington’s decision-making process on immigration. Of course geopolitical considerations are—and should be—taken into account when the federal government makes immigration policy.