The advent of World War II saw and end of the period of economic turmoil and massive unemployment known as the Great Depression, and thus was a time of increased opportunity for many of the nation's citizens and immigrants, but the experiences of some groups during and following the war were far less positive than others. Some of this was due to the different histories that different immigrant groups had in the country, as well as the different roles that various nations played in the war itself, but often the source for the treatment of different ethnic groups was all too similar and all too simple -- racism and ethnocentrism that made the white Americans "true" citizens while others were labeled as outsiders, and those that didn't belong. The Japanese suffered the worst during World War II; even families that had been in the country for generations and many decades were viewed with a great deal of suspicion and mistrust, and this eventually coalesced into the creation of Japanese internment camps where families were forced to leave their homes and business (Takaki 2008). Many Japanese-Americans dies in these camps, as hygiene and healthcare was poor, living space was incredibly cramped, and access to food and clean water was severely limited (Takaki 2008). Chinese-Americans fared slightly better, though a general suspicion of all Asian-Americans and Asian immigrants was typical during this period, and employment opportunities outside of the ethnically Chinese pockets that existed in many major cities were severely limited due to overt racist practices, even after the immigration limits enacted in earlier decades had been largely rescinded (Takaki 2008).

Blacks, Mexicans, and Native Americans -- groups that were more traditionally and more prevalently (if only due to their larger numbers and longer history in the country) labeled as "outsiders" to mainstream American culture and society -- were also impacted significantly by World War II. Blacks actually achieved greater levels of equality, showing a great reluctance to take part in the war and being quite outspoken in their demands...

The Mexican experience was highly similar to their experience during World War I, at least insofar as they were seen as an abundant source of cheap labor especially in the agricultural industry, and the bracero program that began during this period is evidence of this (Vogel 2004). Huge numbers of Native Americans enlisted in the United States' military; the Iroquois Confederacy had declared war on Germany and never signed a treaty, making them especially eager to fight n World War II, and many other tribes also saw as much as a quarter or more of their men enlist to fight overseas (Morgan 1995). While Mexicans were again rejected after the war, however, Native Americans were seen as more fully integrated following their military service even though not all of them wanted this, and while they managed to straddle their Native and mainstream worlds Mexicans were relegated back to their lives as poor itinerant laborers or shipped back to their impoverished homes (Morgan 1995; Vogel 1994).
The different treatment of various ethnic groups in the United States during World War II occurred for a variety of reasons. Ultimately, however, it was the prevailing view of these individuals by mainstream society prior to the war that determined the trajectory of any change.