The New York Times summarizes how we got here:
In May, Attorney General Jeff Sessions installed a “zero tolerance” immigration policy, in which the government sought to jail and prosecute every adult who crossed the border illegally.
For months, migrant families were separated as adults were placed in detention, and their children were sent to shelters across the country. Widespread outrage led the Trump administration to end the practice.
In July, the Trump administration lost its bid to persuade a federal court to allow long-term detention of migrant families.
If you can’t win in court, get a feckless GOP Congress to change the rules for you — if you can.
Two weeks ago the Trump administration proposed a new regulation to allow the government to sidestep the 1997 consent decree known as the Flores agreement and detain children with their parents while their cases are being considered by immigration courts.