As the Trump administration’s aggressive immigration crackdown has swept across the U.S., countless people have been documenting federal agents’ actions, warning neighbors of their whereabouts with whistles and car horns, and uploading videos of encounters to social media. Two people who have been on the front lines are Ben and Sam Luhmann, a pair of homeschooled teenage brothers from the Chicago area. They kicked off their work as so-called ICE watchers when “Operation Midway Blitz” began in the Chicago region back in September, and most recently were on the ground in Minnesota documenting immigration enforcement actions in the Twin Cities....