Border Patrol Agent Eduardo Olmos walks near the secondary fencing separating Tijuana, Mexico, and San Diego in June 2016. (AP) (Gregory Bull/AP)
Tore Olsson is a historian at the University of Tennessee, and the author of the award-winning book "Agrarian Crossings: Reformers and the Remaking of the US and Mexican Countryside."

Since his debut on the political stage, Donald Trump has unceasingly pledged to construct a “big, beautiful wall” on the United States’ southern border, believing it would provide an immediate solution to the dilemma of undocumented immigration from Mexico. Toward that end, President Trump has forced a partial government shutdown and Tuesday night sought to persuade the public of the wall’s necessity.

The border wall is unambiguously intended to keep Mexicans out of America. But if Trump wants to stem migration from Mexico, he should also consider keeping Americans out of Mexico.

I’m not thinking of the American retirees, honeymooners or spring-breakers who visit Mexico every year, although these folks undeniably number in the millions. I’m thinking instead of the far more powerful representatives of American business, government and philanthropy who have played a pivotal role in transforming Mexico during the past century, particularly by decimating Mexico’s rural economy — and thereby doing a great deal to stimulate emigration to the United States.

Trump’s plan for the wall has said nothing about this southbound traffic. Yet, as more than 100 years of history reveals, that American involvement in Mexico has played a vital role in spurring northward migration.

In the late 1800s, profit-seeking U.S. businessmen swarmed into Mexico, welcomed by the pro-American dictator Porfirio Díaz. U.S. investors bought up plantations, ranches, oil fields and railroads, and by the turn of the 20th century, they controlled vast swaths of the Mexican economy. Such American domination sowed bitterness among Mexicans and ultimately helped spark the Mexican Revolution of 1910-1917.

That sweeping revolt, which overthrew the Díaz regime and sent many U.S. businessmen home, uncorked a violent social revolution that would claim countless lives. Hundreds of thousands of Mexicans fled north to escape the revolution’s bloodshed in the first major wave of Mexican migration to the United States, and they were welcomed with open arms by southwestern U.S. agribusinesses eager for cheap labor. As a result, Mexicans were legally exempted from U.S. immigration quotas until the 1960s.

During World War II, the U.S. government itself began to recruit Mexican workers to fill agricultural jobs vacated by Americans serving the war effort. The so-called “bracero” (manual laborer) program, a partnership between the U.S. and Mexican governments that began in 1942, transported hundreds of thousands of Mexican men during the war years to work seasonally in states such as California and Texas.

Enormously popular with American agribusiness, the bracero program was extended after the war. By the time it was dismantled in 1964, the program had issued nearly 5 million work contracts to Mexican men. It had also fostered a culture of seasonal migration that continued after the program’s end, even if that migration was no longer legally sanctioned.

As Mexican revolutionary tensions cooled, American businessmen and reformers also began to return. Beginning in 1943, agronomists employed by the Rockefeller Foundation entered Mexico with the goal of modernizing Mexican agriculture. These scientists, intoxicated with the promise of pesticides, hybrid seeds and mechanization, hoped to remake the Mexican farm along the lines of factory farms of the U.S. Midwest. Over the next 20 years, they would have surprising success in doing so — but at tremendous social cost. Millions of Mexican peasant cultivators were displaced by these technological revolutions in agriculture. Those uprooted farmers streamed into Mexican cities looking for work. Finding opportunities there scarce, many headed to the United States.

Beginning in the 1960s, U.S. corporations also established manufacturing plants in Mexican border cities such as Ciudad Juárez to take advantage of cheap, non-union labor while retaining geographical proximity to American markets. This corporate influx spurred a massive internal migration to Mexico’s northern states, as desperate rural people sought better-paying factory work. But because these American-owned maquiladoras — border factories — preferred to hire young women, Mexican men were largely left out in the cold. Many would leave their wives and girlfriends behind to seek undocumented work within the United States.

After Mexico’s economic crash of the 1980s, the U.S. government pressured Mexico to slash regulations, public spending and protectionist tariffs, the latter perhaps best exemplified by NAFTA. That treaty, enacted in 1994, eliminated taxes and tariffs on a wide range of goods, including agricultural commodities. As a result, Mexico was soon flooded by a tidal wave of cheap Midwestern corn, a mass-produced crop subsidized by the U.S. government and sold at a much lower price than the corn grown by Mexican farmers in the languishing countryside. As a result, NAFTA helped bring about a second depopulation of rural Mexico, pushing millions of former peasants into a migrant stream headed north to alleviate this newfound poverty.

The violence of the drug war has heightened the factors that drive Mexicans to emigrate. Yet it is important to remember that Mexican drug cartels are bankrolled by American drug users and armed with American-bought weapons. And those cartels are often fighting Mexican government troops themselves funded and equipped by the U.S. government’s Mérida Initiative of 2007, which has funneled billions of dollars into Mexico to fight the global war on drugs.

For more than a hundred years, Americans in Mexico have been instrumental in fostering Mexican migration northward. But that inconvenient truth is absent in the debate about the border wall. As Trump imagines it, the wall is merely meant to block “dangerous” immigrants from crossing into the United States without papers. The impact of American corporations, commodities, guns and dollars in driving this migration is rarely addressed.

If future U.S. involvement in Mexico looks anything like that of the past century — and there’s little reason to think it won’t — a major impetus for migration will continue to exist. A border wall may hamper that movement but is unlikely to end it. As long as Americans continue to flood into Mexico and reshape its economy, the drive for Mexicans to enter the United States will remain higher than any wall.