Porous Borders is firmly situated in the interdisciplinary scholarship on borderlands and contributes to this literature by including the experiences of Chinese migrants who lived on both sides of the U.S.–Mexican border, as well as those of Mexicans, African Americans, English-speaking whites, and various First Nations peoples.

Much of the first portion of the work explores El Paso in the late nineteenth century, when the railroad transformed it from a small settlement to a bustling city. Capitalism put people, commodities, and ideas into continual motion, with El Paso as a hub. Lim emphasizes the multiracial and multiethnic character of El Paso, which was populated by Mexicans (classified by the United Stateas census as white), Chinese, African Americans, and English-speaking whites, who amicably resided among each other. African Americans from the South moved to El Paso seeking to escape Jim Crow and racial violence. Likewise, Chinese immigrants and citizens on both sides of the border established thriving businesses away from the anti-Chinese laws and rancor of the Pacific Coast and Southwest states. United States and Mexican immigration officials allowed for easy crossings by Mexicans, Blacks, and English-speaking white merchants, businessmen, laborers, and migrants. El Paso, as presented by Lim, provided a quasi-sanctuary of congenial relationships as various racial and ethnic groups intermarried, lived side by side, and together engaged in public and private amusements.

Such tolerance, however, ebbed and flowed, and in the end would not last, as white elites soon sought to establish a racial hierarchy that did not easily map onto people's self-identity or history. Although Porous Borders provides rich details regarding the multiracial and ethnic social history of El Paso, it is a bit vague regarding when, how, and why such racial dynamics shifted and broke down. When discussing the rise of English-speaking white supremacy in the first quarter of the twentieth century, such action is broadly attributes to “elites.” This is not entirely edifying. If a vibrant multiracial city made the wheels of capitalism turn, why was it so important to elites to impose racial hierarchies? Likewise, one must wonder whether non-elite English-speaking whites in El Paso harbored their own desires to reap the wages of whiteness.

Moving away from El Paso, one of Porous Borders’s achievements is providing part of the rich history of Chinese immigrants and those of Chinese ancestry in Mexico. Unable to enter the United States because of the Chinese Exclusion laws, Chinese immigrants settled in Mexico. Some prospered as entrepreneurs, opening laundries, restaurants, and stores. Men of Chinese ancestry also married Mexican women without significant problems. Yet, the Mexican Revolution dramatically changed such racial and ethnic tolerance. Lim argues that Mexican revolutionary racial ideology embraced the mestizo (a person of mixed Spanish and Indigenous ancestry). However, after the new Mexican government took power, it drafted restrictive immigration laws that essentially excluded Chinese people from immigrating. Likewise, it would eventually bar the immigration of African Americans.

During and after the revolution, Chinese immigrants in Mexico provided easy scapegoats, and Lim recounts numerous events involving mob violence against them. As some Chinese people fled Mexico, they turned North hoping to enter the United States. At times, United States immigration officials allowed for temporary refuge, but they typically refused such sanctuary. With few options available, some Chinese entered the United States illegally. If caught, one might languish in prison or be deported back to Mexico or even China.

A particularly harrowing episode that Lim explores involved Chinese immigrants and merchants in Mexico who offered United States army troops assistance in the Punitive Expedition (1917) in which the United States attempted to capture Pancho Villa and other Mexican revolutionaries who made incursions onto United States soil. Mexican-Chinese immigrants supplied troops with crucial goods and services, allowing them to survive in desert conditions. When General Patton withdrew his troops, he insisted that these Chinese helpers be allowed to enter the United States. Although the State Department, the Department of Labor, and immigration officials sought to deny Patton's request, he persevered, as abandoning them in Mexico would result in their murder. Lim movingly depicts the army's evacuation of these people in refugee caravans under military protection, and the subsequent legal limbo that the United States government created for them. She uses this episode to demonstrate how different parts of the United States federal government clashed with one another over immigration issues, belying the idea that a monolithic state existed in the United States at the federal level.

As Mexico hardened its borders, so too did the United States Congress pass increasingly strict immigration laws and build the apparatus to enforce such laws. Mexicans were exempt from the 1921 and 1924 National Quota Laws, but they were subject to the long-standing “likely to become a public charge” provision. This vested enormous authority in immigration officials to deny entry to migrants. Such officials had long looked the other way at the southern border, allowing Mexican workers to cross because of pressure from agricultural interests invested in cheap migrant labor. However, by the 1920s, Mexican migrants’ entry into the United States became increasingly difficult. In 1929, Congress made it a criminal offense to cross the boarder without passing through an immigration station, and the federal government opened La Tuna, a prison just north of El Paso where those Mexicans who had illegally crossed the border were jailed. Being Mexican in the borderlands became indelibly associated with criminality, and El Paso, once a major entry point into the United States, became a busy exit point.

Porous Borders provides the reader with a number of edifying, valuable, and significant histories. There are, however, some holes. Given the enormous rupture that the Mexican revolution created in Mexico's racial tolerance, as demonstrated by new immigration laws, the author might have further contextualized and analyzed why the revolution produced such rampant xenophobia. Further, the discourse that Mexican officials and others used in scapegoating the Chinese was substantially similar to that deployed in the United States. One wonders the extent to which it was a United States export. When admiring the free flow of ideas, people, and things across borders, the currency of hatred must not be undervalued.