Meanwhile, in the Democrats’ liberal base, the mood toward the caravan is positively sympathetic. The caravan’s slogan, “People without borders,” chimes with the rising sentiment among liberals that border enforcement is inherently illegitimate, and usually racist, too.

But understand what’s at stake: The theory behind the caravans—this latest, and its smaller predecessors over the past 15 years—is that Central Americans have valid asylum claims in the United States because of the pervasive underemployment and gang-violence problems in their countries. If that claim is true, that is a claim shared not only by the thousands in the current caravan, but by the millions back home. A 2013 Pew survey found that 58 percent of Salvadorans would move to the United States if they could. The seven countries of Central America together have a population of some 45 million, or about the same as Mexico’s back in 1970, when the mass migration from that nation began.

Photos of the Central American immigrant caravan

Things happen much faster in the 2010s than they did in the 1970s. When Germany temporarily suspended its border rules in August 2015, almost 1 million migrants surged into the country within the next four months. That surge continued into 2016. Its political effects linger still: It was crucial to the British vote to quit the European Union, to the election of a reactionary government in Poland, to the political revival of Viktor Orbán in Hungary, and to the collapse of center-left parties in France, Italy, Sweden, and Germany.

Trump’s election owes something to the surge across the U.S.’s southern border in the summer of 2014. Tens of thousands of women and children crossed the border in only a few weeks. Many of those who entered in 2014 remain in the United States to this day, even after their cases have been negatively adjudicated, because they have disregarded their removal orders and vanished into the vast U.S. population of unauthorized immigrants.

The strong U.S. job market is again attracting low-wage workers. After a dip in 2017, illegal crossings of the southern border in 2018 have returned to their levels of 2016—and are running well ahead of 2015. If the thousands of people in the caravan successfully cross the border, lodge asylum claims, and are released into the U.S. interior pending adjudication, many more seem likely to follow.

Why wouldn’t they? More than 60 percent of the population of Honduras lives in poverty, according to the World Bank, and very nearly 60 percent does so in Guatemala. While rates of crime and violence have declined in both countries since 2014, they remain appalling by world standards.

For Trump, the caravan represents a political opportunity. Here is exactly the kind of issue that excites more conservative Americans—and empowers him as their blustery, angry champion.