During World War II, the United States forcibly relocated and incarcerated about 120,000 people of Japanese descent in ten concentration camps operated by the War Relocation Authority (WRA), mostly in the western interior of the country. About two-thirds were U.S. citizens. These actions were initiated by Executive Order 9066, issued by President Franklin D. Roosevelt on February 19, 1942, following Imperial Japan's attack on Pearl Harbor on December 7, 1941. About 127,000 Japanese Americans then lived in the continental U.S., of which about 112,000 lived on the West Coast. About 80,000 were Nisei ('second generation'; American-born Japanese with U.S. citizenship) and Sansei ('third generation', the children of Nisei). The rest were Issei ('first generation') immigrants born in Japan, who were ineligible for citizenship. In Hawaii, where more than 150,000 Japanese Americans comprised more than one-third of the territory's population, only 1,200 to 1,800 were incarcerated.

Internment was intended to mitigate a security risk which Japanese Americans were believed to pose. The scale of the incarceration in proportion to the size of the Japanese American population far surpassed similar measures undertaken against German and Italian Americans who numbered in the millions and of whom some thousands were interned, most of these non-citizens. Following the executive order, the entire West Coast was designated a military exclusion area, and all Japanese Americans living there were taken to assembly centers before being sent to concentration camps in California, Arizona, Wyoming, Colorado, Utah, Idaho, and Arkansas. Similar actions were taken against individuals of Japanese descent in Canada. Internees were prohibited from taking more than they could carry into the camps, and many were forced to sell some or all of their property, including their homes and businesses. At the camps, which were surrounded by barbed wire fences and patrolled by armed guards, internees often lived in overcrowded barracks with minimal furnishing.

In its 1944 decision Korematsu v. United States, the U.S. Supreme Court upheld the constitutionality of the removals under the Due Process Clause of the Fifth Amendment to the United States Constitution. The Court limited its decision to the validity of the exclusion orders, avoiding the issue of the incarceration of U.S. citizens without due process, but ruled on the same day in Ex parte Endo that a loyal citizen could not be detained, which began their release. On December 17, 1944, the exclusion orders were rescinded, and nine of the ten camps were shut down by the end of 1945. Japanese Americans were initially barred from U.S. military service, but by 1943, they were allowed to join, with 20,000 serving during the war. Over 4,000 students were allowed to leave the camps to attend college. Hospitals in the camps recorded 5,981 births and 1,862 deaths during incarceration.

In the 1970s, under mounting pressure from the Japanese American Citizens League (JACL) and redress organizations, President Jimmy Carter appointed the Commission on Wartime Relocation and Internment of Civilians (CWRIC) to investigate whether the internment had been justified. In 1983, the commission's report, Personal Justice Denied, found little evidence of Japanese disloyalty and concluded that internment had been the product of racism. It recommended that the government pay reparations to the detainees. In 1988, President Ronald Reagan signed the Civil Liberties Act of 1988, which officially apologized and authorized a payment of $20,000 (equivalent to $53,000 in 2024) to each former detainee who was still alive when the act was passed. The legislation admitted that the government's actions were based on "race prejudice, war hysteria, and a failure of political leadership." By 1992, the U.S. government eventually disbursed more than $1.6 billion (equivalent to $4.25 billion in 2024) in reparations to 82,219 Japanese Americans who had been incarcerated.

The United States Government had previously employed civilian internment policies in a variety of circumstances. During the 1830s, civilians of the indigenous Cherokee nation were evicted from their homes and detained in "emigration depots" in Alabama and Tennessee prior to the deportation to Oklahoma following the passage of the Indian Removal Act in 1830. Similar internment policies were carried out by U.S. territorial authorities against the Dakota and Navajo peoples during the American Indian Wars in the 1860s. In 1901, during the Philippine–American War, General J. Franklin Bell ordered the detainment of Filipino civilians in the provinces of Batangas and Laguna into U.S. Army-run concentration camps in order to prevent them from collaborating with Filipino General Miguel Malvar's guerrillas; over 11,000 people died in the camps from malnutrition and disease.

After the United States entered World War I in 1917, roughly 6,300 German-born residents of the United States were arrested, with 2,048 of those residents being incarcerated at two U.S. Army bases, where they remained interned until 1920. However, these policies only targeted a small fraction of German-born Americans and did not apply to German-American U.S. citizens.^{[citation needed]}

Due in large part to socio-political changes which stemmed from the Meiji Restoration—and a recession which was caused by the abrupt opening of Japan's economy to the world economy—people emigrated from the Empire of Japan after 1868 in search of employment. From 1869 to 1924, approximately 200,000 Japanese immigrated to the islands of Hawaii, mostly laborers expecting to work on the islands' sugar plantations. Some 180,000 went to the U.S. mainland, with the majority of them settling on the West Coast and establishing farms or small businesses. Most arrived before 1908, when the Gentlemen's Agreement between Japan and the United States banned the immigration of unskilled laborers. A loophole allowed the wives of men who were already living in the US to join their husbands. The practice of women marrying by proxy and immigrating to the U.S. resulted in a large increase in the number of "picture brides."

As the Japanese American population continued to grow, European Americans who lived on the West Coast resisted the arrival of this ethnic group, fearing competition, and making the exaggerated claim that hordes of Asians would take over white-owned farmland and businesses. Groups such as the Asiatic Exclusion League, the California Joint Immigration Committee, and the Native Sons of the Golden West organized in response to the rise of this "Yellow Peril." They successfully lobbied to restrict the property and citizenship rights of Japanese immigrants, just as similar groups had previously organized against Chinese immigrants. Beginning in the late 19th century, several laws and treaties which attempted to slow immigration from Japan were introduced. The Immigration Act of 1924, which followed the example of the 1882 Chinese Exclusion Act, effectively banned all immigration from Japan and other "undesirable" Asian countries.^{[citation needed]}