Fences have existed along some parts of the U.S.-Mexico border for decades. In the early 1990s, Washington began to construct more substantial barriers in urban areas, many of which were made with surplus metal runway mats from the Vietnam War. More sophisticated fencing techniques did not appear until 1995, when Sandia National Laboratories created a barrier three layers deep that was designed to slow intruders until border patrol agents could respond to the breach. In this scheme, the layer closest to the foreign country is a thick metal wall, separated from the middle layer — a metal mesh fence — by a well-lit open area blanketed with technological surveillance, including cameras, thermal imaging and an array of sensors. Then, in areas most prone to heavy traffic, a low fence forms the third and innermost layer.
In 2006, the Secure Fence Act sought to extend existing fences along the border. Yet even with the additions, there are still hundreds of kilometers of gaps in the nearly 3,200-kilometer (2,000-mile) border. Lawmakers have repeatedly proposed measures that would fund fence-building in these areas, but none have been approved because of the serious doubts that remain on fences' effectiveness in deterring illegal border crossings. Filling in the rest of the border (with the exception of a 322-kilometer stretch of land in southwest Texas) is estimated to cost somewhere between $7 billion and $10 billion.
Trump, of course, has vowed to completely seal the U.S.-Mexico border by building a wall. One of the biggest problems with this proposal is that the flow of illegal immigrants and contraband between the two countries is not a simple matter of physical security or immigration law. Rather, the cross-border movement of goods and people is driven by formidable economic forces that are powerful enough to overwhelm any barrier. Border barriers can redirect the illicit flow of people or goods, but they cannot stop it. Driven by the prospect of striking it rich, smugglers have come up with any number of creative means to go over, under or through walls. They are constantly coming up with new ways to hide contraband. In fact, far more drugs cross the U.S.-Mexico border through official checkpoints than are smuggled through the empty expanses of desert on either side.
Consequently, Mexican drug cartels spend a great deal of effort fighting for control of walled border-crossing cities (referred to as "plazas" in Spanish). Massive amounts of illegal trade pass through these towns, and the organizations that control them can collect a tax (or "piso") on the smuggling activities taking place there. Cartels have expended immense blood and treasure to capture and hold cities such as Nuevo Laredo, Reynosa, Juarez and Tijuana — all of which have had walls running through them for decades.