A national border, as Noam Chomsky once put it, is "artificially imposed and, like those many other borders imposed by external powers, it bears no relationship to the interests or the concerns of the people of the country." Rather, borders are largely a curse for workers.
A good illustration can be found in an examination of the United States-Mexico border.
The border's current shape was drawn in 1853, with much of it fashioned during the bloody, two-year conflict between U.S. and Mexico from 1846 to 1848. Until 1929, the border was unguarded, with immigrants having a choice between paying fees and undergoing examinations at the scattered ports of entry, or simply crossing wherever they could. The Immigration Act of March 4th, 1929, proposed by a white supremacist Senator from South Carolina, made informal entries a crime.
Yet despite this new law, the number of apprehensions along the border remained static until the end of World War II. When the majority of the 12 million military personnel came home and re-entered the workforce, border apprehensions rose, culminating in the "wetback airlift" of 1954, when more than one million Mexican immigrants in America without documentation were deported. On the surface, this seems a straightforward case of replacement workers being ousted when the original workforce returned home, but something else occurred during this time that muddles that thesis.
In 1942, with the U.S. workforce at war, the U.S. and Mexico agreed on terms for the bracero program, allowing millions of Mexican immigrants to cross the border on short-term work permits, mostly for agricultural work. But rather than ending the program after the war, it continued until 1964. Why? Because in 1951, farm owners, who'd benefited from the poorly protected (and, as such, cheaper) workforce, rallied support to extend the program. It worked, and the program was formalized as Public Law 78.
The law effectively created two separate workforces from which farm owners choose from: one made of local workers, with protections afforded through unionization, and one was from Mexico, and couldn't go on strike or renegotiate wages. While employers weren't supposed to use "braceros" to break strikes, few followed those rules, which had the effect of keeping wages down for all workers. This race-to-the-bottom between separated workforces, a fight that inherently benefits employers, happens when workers either have different sets of rights, or are physically kept apart by a militarized border.
The next big leap in security occurred with Operation Gatekeeper, implemented under the Bill Clinton presidency to "restore the integrity and safety" of the border. The measure expanded the U.S. Border Patrol's budget enough to hire 200 more agents to patrol a 14-mile stretch south of San Diego. Subsequent measures under Clinton strengthened U.S.-Mexico border security. A video passed around online last year showed Clinton talking about how border security was necessary because illegal immigrants were taking jobs from Americans, and nearly every administration has followed in its footsteps, with the number of border agents peaking during Barack Obama's presidency. Donald Trump's border wall, whatever shape it takes, is simply the next extension of decades of U.S. policy.
It's worth considering why security measures were heightened in the mid-'90s, besides the empty "integrity and safety" rhetoric that Clinton espoused at the time. An answer likely lies in the creation of the trilateral trade block the North American Free Trade Agreement, which went into effect on the first day of 1994.
Among other things, NAFTA removed tariffs on imports and exports across Canada, the U.S., and Mexico. The agreement, theoretically, would create more jobs, as capital flowed without border restrictions. But, as NAFTA essentially erased the border for companies, Operation Gatekeeper's blockade further restricted the movement of workers. In response to this stricter separation, unrestricted businesses were allowed, and thus incentivized, to lower their production costs by using the cheapest, least protected labor possible. This was found south of the border.
The most blatant symbol of NAFTA's effect has been the rise of the maquiladora, factories in Mexico that manufacture or produce items exclusively for cross-border trade. In its most-straightforward form, maquiladoras import raw material from the U.S., assemble it in Mexico using a Mexican workforce, and sell it back to the U.S. They tend to be physically located near the border, so as to lower shipping costs. They can also be owned by foreign companies, and so, many are owned by U.S. companies, making the factory's entire existence exclusively about lowering labor cost.
Maquiladoras, then, are the obvious result of businesses being allowed to cross national borders with ease, while workers are heavily restricted. A separated workforce gives owners a tremendous amount of power when determining how to compensate their workers. In the macro, that could mean setting up production across the border in Mexico, where six-day workweeks are legal. But in the micro, it takes the form of the Target in Austin, Texas, that, in 2007, was caught paying its undocumented janitorial staff $4.35 an hour and locking them in the stores overnight.
Businesses will always pay their workers as little as they can get away with. That's at the core of how they're structured, and why government-instituted worker protections exist. But as long as national borders are strictly enforced, while technology continues to afford the globalized movement of trade, workers will continue to be forced to fight one another to the bottom.