On Friday morning, President Trump declared a national emergency at the southern border, in order to access billions of dollars in funding for the construction of a wall. In doing so, Trump defied the authority of Congress, which had settled on a bill to fund the government that included limited funding for border fencing. Another major issue for congressional negotiators was the number of immigrants held in detention. While Trump has been ramping up detention numbers, including of people seeking asylum, Democrats have been pushing back. Ultimately, lawmakers agreed to fund just more than forty-five thousand beds, which would decrease the number of detained immigrants by about seventeen per cent. The debate over how many people the U.S. should detain has skirted a larger question: Why does America detain so many illegal immigrants and asylum seekers in the first place?

To discuss this question, and to get some historical perspective on U.S. relations with Mexico, I spoke by phone earlier this week with Ana Raquel Minian, an associate professor of history at Stanford and the author of the book “Undocumented Lives: The Untold Story of Mexican Migration.” During our conversation, which has been edited and condensed for clarity, we discussed what the U.S. can do to create a more stable Mexico, the factors that cause net migration rates to go up and down over time, and why the U.S. did not always find it necessary to lock up people seeking asylum.