It was in this environment, against the backdrop of rising nativist sentiment, that the Clinton administration launched Operation Gatekeeper on Oct. 1, 1994. The operation promised “prevention through deterrence.” It beefed up enforcement in urban areas around San Diego and pushed migration to rural, less visible areas. Operation Gatekeeper also disrupted border communities by instituting checkpoints, boots on the ground, technology and border wall infrastructure.

The Border Patrol’s public rationale was that making it harder to cross the border would mean fewer people would try to come to the United States. Policymakers theorized that people might not cross the border at all if it meant making a risky, potentially deadly journey.

The border has not always been militarized. Even before the United States and Mexico made any claim to the land, indigenous peoples like the Kumeyaay in California, the Tohono O’odham in Arizona and the Lipan Apache of New Mexico and Texas inhabited the Southwest for generations. Their descendants now must contend with physical structures that partition the lands their ancestors traversed without any barriers.

The location of the border itself changed considerably during the 19th century, with the United States occupying increasingly larger parts of northern Mexico. The Border Patrol was not created until 1924 to enforce immigration restrictions, initially without a clear mandate from Congress. Nonetheless for decades afterward, there remained few physical barriers between the United States and Mexico.

That began to change in the late 1960s, when the Nixon administration began fighting its “war on drugs” at the border through “Operation Intercept,” which was run by future anti-immigrant sheriff Joe Arpaio. The piecemeal enforcement of those years became more formalized under Ronald Reagan. In 1986, Congress reformed immigration laws to allow people in the United States to adjust their legal status. The trade-off was increased immigration enforcement and employer sanctions that led to worksite raids and migrant detention.