The "Remain in Mexico" program is a key part of the Trump administration's plan to turn back a crush of migrants at the southern border, and it's a historic shift in how the asylum system works.
President Trump announced the secretary's departure amid a 10-year high in illegal border crossings. The two reportedly clashed as Nielsen was unable to stop flows of migrants entering the U.S.
Two memos, signed by Secretary John Kelly, greatly expand the number of immigrants prioritized for removal. The rules do not affect "Dreamers" — people brought to the U.S. illegally as children.