Like Nelcy and her daughters, the new arrivals from Central America are coming in much sicker, after being held far longer than ever before in bare-bones government detention facilities never intended for children. Asylum seekers bottled up in Mexico are jumping fences and throwing rocks at officers, who are firing tear gas to push them away. Hundreds of migrants have been released on city streets in recent weeks, uncertain of where to go. Two sick migrant children have died while in custody.

A crisis of the kind President Trump has long warned of is beginning to take shape along the country’s 1,900-mile border with Mexico. A border security network built over a period of decades to handle large numbers of single men has in the past several years been inundated with women and children, and as the number of families has peaked in recent months, the system has increasingly been unable to accommodate all of them.

Much of the growing chaos, say many of those who work along the border and in some of the government’s own security agencies, is a result of a failed gamble on the part of the Trump administration that a succession of ever-harsher border policies would deter the flood of migrants coming from Central America.

It has not, and the failure to spend money on expanding border processing facilities, better transportation and broader networks of cooperation with private charities, they say, has led to the current problems with overcrowding, health threats and uncontrolled releases of migrants in cities along the border.