Good neighbors can make good fences: but when it comes to Mexico, the United States isn't being very neighborly.

• Author: Mangu-Ward, Katherine
• Position: POLITICS

GOOD FENCES MAKE good neighbors, or so Robert Frost reminds us in his annoyingly overused and frequently misquoted high school literature class staple. The poem that made the adage famous actually offers a more ambiguous take on the utility of border barriers than its signature line would suggest, with the speaker musing: "Before I built a wall I'd ask to know / What I was walling in or walling out, / And to whom I was like to give offence."

The question of what exactly is being walled in or walled out by Donald Trump's barrier--he issued commands for the "immediate construction of a physical wall on the southern border" in a January 25 executive order--is trickier to answer than it initially appears. The short answer, illegal immigrants, is an unsatisfactory one, in part because so many other goals tend to get lumped in once the policy rationalization process gets rolling, including drug interdiction, terrorism prevention, and tariff enforcement.

The question of who will be offended is easier. From Trump's unflattering remarks about Mexican immigrants while announcing his candidacy in June 2015 to his ongoing insistence that Mexico will pay for the wall, much offense has been given, and much taken.

During the campaign, Trump flew to visit Mexican President Enrique Pena Nieto. Upon returning home, the candidate claimed that they had discussed the wall but not who would pay for it--an assertion his counterpart denied. Shortly after his inauguration, tensions built around a planned visit by the Mexican president to the north. "If Mexico is unwilling to pay for the badly needed wall," Trump tweeted, "then it would be better to cancel the upcoming meeting." When Pena Nieto did just that, Trump made it clear that he would consider garnishing some of the $26 billion in annual remittances from the U.S. to Mexico.

The Associated Press also reported the following astonishing threat by Trump, gleaned from (disputed) transcripts of a phone conversation between the two men: "You have a bunch of bad hombres down there. You aren't doing enough to stop them. I think your military is scared. Our military isn't, so I just might send them down to take care of it." Simultaneously on the table during that period: A 20 percent tax on goods at the Mexican border, though that idea was withdrawn almost as quickly as it was proposed.

When Trump addressed a joint session of Congress on February 28, he reiterated his intention: "We will soon begin the construction of...