After nightfall on Sunday, hundreds of migrants stepped across the Rio Grande and into El Paso, a caravan of people mainly from Nicaragua whose crossing was among the largest in recent years along the West Texas border.

Their arrival en masse into the United States surprised even those in El Paso, which has in recent months found itself overwhelmed by a steady stream of many migrants from Central and South America, more than 50,000 people in October alone.

Like the migrants from Venezuela who flooded into El Paso this year, those arriving from Nicaragua cannot be rapidly expelled under a pandemic-era public health policy known as Title 42, which federal authorities employ with migrants from other countries, such as Mexico.

And so the scenes unfolding in El Paso offered a preview of the challenges that border officials could soon face all along the southern border after the policy comes to an end, as it is expected to, absent court intervention, next week.