TAPACHULA, Mexico — About 1,500 migrants, mainly from Central and South America, formed a caravan Wednesday in southern Mexico, hoping to walk or catch rides to the U.S. border. Some say they are hoping to reach the United States before Donald Trump’s inauguration in January, thinking it might be more difficult after that. They began walking from Tapachula, near the border with Guatemala, where thousands of migrants are stranded because they do not have permission to cross further into Mexico. What are migrant caravans? Migrant caravans began forming in 2018, and they became a final, desperate hope for poorer migrants who do not have the money to pay smugglers. If migrants try to cross Mexico alone or in small groups, they are often either detained by authorities and sent back to southern Mexico or, worse, deported to their home countries. In that sense, there is safety in numbers: It is hard or impossible for immigration agents to detain groups of hundreds of migrants. So, police and immigration agents often try to pick off smaller groups and wait for the main body of the caravan to tire itself out. Usually, the caravans stop or fall apart within 250 kilometers (150 miles). What are the obstacles? There is no safety in numbers against threats, extortion or abduction by drug cartels in Mexico, which have become heavily involved in migrant trafficking. The cartels charge migrants or their smugglers for permission to cross their territories along the border. In addition, the gangs often kidnap migrants, hold them in terrible conditions or torture them until they call relatives to send money for their release. The biggest obstacle, though, is the searing heat, dehydration and distance — it is over 1,770 kilometers (1,100 miles) from Tapachula to the nearest border crossing at Matamoros, across from Brownsville, Texas. And that is the shortest, but also one of the most dangerous routes. It would mean 16 days of walking, with no rest stops. And many of the migrants come with their children. Why do they set out? Since migrants usually cannot find work to support themselves in Tapachula, most of the foreigners trapped there are desperate to leave. Some feel a sense of urgency. “It is going to be more difficult” after Trump’s January 20 inauguration. “That’s why we are going — in hopes of getting an appointment quicker, so we are able to cross before he takes office,” said Yotzeli Peña, 23, a migrant from Venezuela. “That would be easier.” Weren’t there changes to keep caravans from forming? This year, in a bid to stop people from gathering at the border to claim asylum, the U.S. government expanded areas where migrants can apply online for appointments to enter the United States to a large swath of southern Mexico. The CBP One cellphone app was instituted to make asylum claims more orderly. About 1,450 appointments are made available daily, encouraging migrants to get an appointment before they show up at the border. But the service was available only in northern and central Mexico. By extending the app south to Tapachula, officials hoped it would stem the rush north. But some migrants still want to be close to the border so that if they do get one of the cherished appointments, they can get to it quickly and not risk missing it. Trump has promised to end the app, reduce legal pathways to the U.S. and organize mass deportations. Do caravans ever reach the border? The biggest caravans formed in 2018 and 2019, and back then Mexican officials helped some of the migrants by arranging buses to border cities. But that created a backlash in those communities. Groups from those original caravans eventually reached the border. In caravans since then, most participants have sought out as many hitchhiking or paid rides as they can and often swarm empty trucks to hitch a ride on empty freight platforms. But that has become much harder as Mexican authorities discouraged buses, taxis and trucks from stopping to pick up migrants. In recent years, authorities have eventually offered temporary transit permits to dissolve the caravans.