President Trump has since backed off his threat, but as border officials scramble to deal with an unprecedented flow of migrants, there are disruptions at the border and increasingly long wait times.
Three possible factors account for the surge of migrants at the border: economics, social media and the Trump administration's own tougher immigration policies.
Eight steel-and-concrete mock-ups of border wall prototypes were put up near the San Diego-Tijuana border in 2017. They were supposed to represent the "next generation" of border barriers.
At the San Diego-Tijuana border, journalists covering the asylum-seekers are being pulled into questioning by customs and border agents. Lawyers and volunteers are facing the same scrutiny.
A temporary shelter has been contracted south of Miami to house more than 2,000 migrant children. It is the nation's only corporate, for-profit migrant youth shelter.
U.S. service members will install wire barriers and help monitor the border. However, some in Congress have questioned whether it's a good use of active-duty troops.
U.S. Customs and Border Protection authorities say they fired tear gas to deter about 150 Central American migrants from climbing over or under the U.S.-Mexico border fence into California.
In June, then-Attorney General Jeff Sessions announced a ban on asylum-seekers fleeing gang violence or domestic abuse. A federal judge on Wednesday blocked that ban, saying it had "no legal basis."
Hundreds of migrant families are being released from immigration custody and are being housed, temporarily, in shelters, hotels and churches across the southwest border region.