Here’s a look at what is happening to migrant families on the border, and what’s behind it.

Is there a policy to separate parents from their children at the border?

The administration did not announce a blanket policy to separate families, but its prosecution policy is having that effect. Mr. Sessions said his department would criminally prosecute everyone who is found to have illegally entered the United States. Children are not allowed to remain with parents who are in custody in the criminal court system, so if a mother or father is apprehended with a child, the administration says, the minor must be taken from the parent.

Is the separation required by law?

The administration says its hands are tied by a 1997 court settlement and later court rulings governing the settings in which detained children can be held. But separating parents from children is only one of several options the administration has for meeting the requirements, even while carrying out its prosecution policy. The White House has also said that longstanding laws require the separations, but experts say there are no such laws.

Is the administration deliberately breaking up families?

Administration officials say the aim is to protect the border and uphold the law through new measures to deter illegal immigration.

Other motivations: Mr. Sessions has said the asylum system is overwhelmed with people making frivolous claims, and Mr. Trump, according to administration officials, had been demanding that families be broken up to stanch the flow of Central Americans to the border. The majority of apprehended migrants come from Honduras or El Salvador, two countries wracked by violence. Children there are often targeted for recruitment by gangs, prompting their families to seek safe haven in the United States.

Are families seeking asylum at the border being separated?

The “zero tolerance” policy is supposed to apply only to people who enter the country illegally. Presenting yourself at a port of entry and requesting asylum is not illegal. But these legal asylum seekers are generally taken into detention while their cases are processed, which can take months, and there are reports that those families are now being separated in the same way that illegal-entrant families are. Complicating the issue, many families that enter illegally request asylum once they are caught.