On Monday morning, news reports strained to account for everything that happened on Sunday at the border crossing that connects San Diego with Tijuana. One comprehensive report said: “Weeks of growing tensions in Tijuana over thousands of migrants who have poured in hoping to enter the United States boiled over Sunday when a group of them rushed the border, prompting U.S. Border Patrol agents to fire tear gas to disperse them.” Another had it this way: “A peaceful march by Central American migrants waiting at the southwestern United States border veered out of control on Sunday afternoon, as hundreds of people tried to evade a Mexican police blockade and run toward a giant border crossing that leads into San Diego.”

Photographs and videos told their own stories: the border crossing completely shut down; U.S. soldiers and Customs and Border Protection agents standing shoulder to shoulder; young men climbing a border fence; Mexican police officers running in riot gear; gas clouds drifting; a mother wrangling two children, one of them barefoot, while fleeing a menacing white plume. Apparently, no migrants managed to cross the border into the U.S. According to the Times, Mexican authorities said that they’d arrested thirty-nine people, and “that those who had attacked federal police would be deported.”

The situation that brought about Sunday’s events is a tangle of causes and effects. Thousands of members of a migrant caravan that crossed Mexico this fall are currently camped out in Tijuana. The caravan came together based on the idea that crossing Mexico in a large group would be safer and more effective than trying to go it alone or enlisting the services of smugglers. It was proved right—but now the caravan’s members have reached the border with the U.S. to find that it is effectively closed to them. Although the migrants have a legal right to request asylum in the U.S., customs officials in San Diego are currently processing only a few dozen claims a day. By travelling in a group, the migrants had also given President Trump an election-season political foil, providing him an excuse to send military personnel to the border and make ridiculous declarations that the country was being “invaded.” He’s happy for this fight.

The caravan has also put pressure on Mexican officials. In Tijuana, the local authorities have been struggling to accommodate the migrants—thousands are now camped out at a sports complex. Meanwhile, Andrés Manuel López Obrador, Mexico’s President-elect, is scheduled to take office on Saturday. Last Saturday, the Washington Post reported that the Trump Administration had “won the support of Mexico’s incoming government” for a plan to make asylum seekers wait in Mexico while their claims move through U.S. courts. What such a “Remain in Mexico” agreement would look like in practice is still unclear, but the worst-case scenario involves migrant refugee camps in Mexican border towns. Desperate groups of migrants scrambling, armed government agents holding them back—on Sunday, these scenes appeared as horrible new developments, difficult to describe. How regular might they become?