As thousands of children were taken from their parents at the southern border during a Trump administration crackdown on illegal crossings, a federal public defender in San Diego set out to find new strategies to go after the longstanding deportation law fueling the family separations. The resulting legal defense that Kara Hartzler would help draft in the coming years — work that continued even after a judge halted the general practice at the U.S.-Mexico border in June 2018 — was unprecedented. It exposed Section 1326 of the Immigration and Nationality Act, which makes it a crime to unlawfully return to...