[Read the latest edition of Crossing the Border, a limited-run newsletter about life where the United States and Mexico meet. Sign up here to receive the next issue in your inbox.]

SAN YSIDRO, Calif. — It was never like this before.

The migrants come now in the middle of the night or in the bright light of day. Men and women arrive by the hundreds, caked with dirt, with teens and toddlers in tow. They jump the small fences in remote parts of Texas, and they gather on the hot pavement at the main border crossing in California. Tired and fearful, they look for the one thing that they pray will allow them to stay in the United States, at least for a while: a Border Patrol agent.

Gone are the days when young, strong men waited on the Tijuana River levees for their chance to wade across the water, evade capture and find work for the summer. These days, thousands of people a day simply walk up to the border and surrender. Most of them are from Central America, seeking to escape from gang violence, sexual abuse, death threats and persistent poverty. The smugglers have told them they will be quickly released, as long as they bring a child, and that they will be allowed to remain in the United States for years while they pursue their asylum cases.

The very nature of immigration to America changed after 2014, when families first began showing up in large numbers. The resulting crisis has overwhelmed a system unable to detain, care for and quickly decide the fate of tens of thousands of people who claim to be fleeing for their lives. For years, both political parties have tried — and failed — to overhaul the nation’s immigration laws, mindful that someday the government would reach a breaking point.