From an earlier answer of mine

Part I

Contrary to received wisdom, there were German and Italian citizens who were interned by the United States during the Second World War. EO 9066 did not specify Japanese, but rather restricted "alien enemies" from residency within designated Military Areas. There were some 11000-15000 people of German and Italian extraction interned by the US during the early war years. However, there were very stark difference between both the process and intents of between the internment of European-born people and that of Japanese and Japanese-Americans. The course and trajectory of Europeans' internment shows how systemic racism acted to their benefit in comparison to the Japanese experience.

First off, the absolute numbers of internees vs the whole population do show a complete disparity between the two internments. Japanese internment not only involved Japanese residents, but also American citizens. By contrast, the internment of German and Italians was usually limited to foreign residents. Such populations could range from recent refugees, residents whose citizenship applications were still being processed, or to those stuck on the wrong side of the Atlantic when the war broke out. Arnold Krammer estimates that of 314,105 German residents living in the US in 1941, less than 11,000 were interned. This stands in stark contrast to the ca.110,000 of the ca. 130,000 Japanese (residents and citizens) who were interned. EO 9066 gave the American military authorities in the Military Areas broad authority to act against enemy aliens, but only the Japanese received the blanket treatment with whole families being interned. Although there were significant logistical concerns over interning the much larger pool of German and Italian communities en masse, very few American leaders in government and the military extended those same concerns to the Japanese population. Most officials instead regarded the size of the Japanese population as an obstacle to be overcome, not a factor that forced a rethinking or softening of the internment process. Thus European internment largely was not because of who the internees were, but rather what they had actually done to earn this suspicion.

There were even greater disparities at an individual level between the Japanese and European cases. More often than not, internment of Europeans was because of an individual's actions. The FBI had busied itself in the run-up to the war in trying to identify potential fifth-columnists. The Emergency Detention Act of 1939 and the Alien Registration Act of 1940 required resident aliens or citizen applicants to identify their country of origin and this allowed federal authorities to trace problematic individuals. Additionally, there was a heightened investigation of the German-American Bund and other institutions such as membership in the NSDAP or other Nazi institutions for foreign-resident Germans. While wartime hysteria led some overzealous detentions and internments, the fears of Nazi subversion were not entirely baseless. A number of pro-Nazi Germans were interred and the camps' newspapers had a pro-Nazi bent. This was quite different than the Japanese experience of internment where expressions of pro-Japanese sentiments in the camps were decidedly minimal.

Moreover, Germans and Italians also enjoyed the benefits of access to legal appeals to their internment. European internees could appeal their incarceration to the Justice Department and around 50% of the Germans detained in the initial wave of internments were released after six months because of this appeals process. Although such bureaucracy was slow and at times humiliating for the European applicants, it was still available. This process of appeal was not available for the Japanese internees. Instead, Japanese-American groups as well as allies in the ACLU had to engage in test cases like Korematsu to question the overall validity of internment and the EOs regarding enemy aliens.

Korematsu, of course, ended in failure and was one of the more infamous SCOTUS decisions in which the majority rationalized discrimination while the dissents recognized the highly questionable constitutional validity of their colleagues' justifications. But the Court's majority opinion shows the dominant attitude of the American political establishment towards the Japanese and it was far from complimentary. Anti-Japanese sentiment pervaded every level of the federal and state government involved in the internment process. Many of the federal employees in charge of the process were indifferent to the plight of Japanese-Americans and federal agencies in charge were understaffed. Racism in this period was not just a matter of violence and slurs, but systemic indifference to inequality. Lieutenant General John L. DeWitt, the chief architect of internment on the West Coast, would claim in Congressional testimony that “a Jap’s a Jap... it makes no difference whether he is an American; theoretically he is still a Japanese and you can't change him.” DeWitt’s own published report late in the war was so racist even by the standards of the day that it was redacted and most of the copies of the original were purposely destroyed until rediscovered in the postwar period. Surveys of American law enforcement showed that while a plurality saw the German population as a more dangerous threat, it was only the Japanese that felt the brunt of law enforcement regulating curfews and other restrictions.

This anti-Japanese sentiment had long been brewing on the West Coast. Anti-Asian racism was a popular bipartisan issue that transected both the Republican and Democratic Parties. Decades of institutionalized racism and social segregation institutionalized the idea that Japanese-Americans lived in an alien world and further reinforced stereotypes of back-stabbing and false patriotism engendered by Pearl Harbor. Politics on all levels of government was primed to think the worst of Japanese-Americans. This meant state governments were quite willing and able to rid themselves as what they saw an unwelcome alien community. In his own testimony to a Congressional subcommittee on Defense in February 1942, California Attorney General Earl Warren would echo DeWitt claiming “when we are dealing with the Caucasian race, we have methods that will test the loyalty of them…but when we deal with the Japanese, we are in an entirely different field and we cannot form any opinion that we believe to be sound.” Despite initial pleas for racial toleration, Governor Culbert Olson would follow the advice of DeWitt and sanction the removal process using the same justification.

Such latent and open hostility was not present in the state's treatment of European aliens. While there were popular calls for the removal of all Axis aliens in military areas, the response of state and federal agencies was to put a break on extending mass restrictions to Europeans. The Tolan Committee, a House subcommittee on the evacuations went to great lengths in its testimony to clear the good name of German and Italian immigrants. San Francisco's mayor Angelo Rossi, for example, pleaded to the subcommittee for the loyalty of Italian and German residents, including the parents of the baseball star Joe DiMaggio. Other aspects of the Tolan hearing went to great lengths to clear the German and Italian communities of wrongdoing, with both congressmen and witnesses contending that assimilation was a core American value and these communities were exemplars of the American experiment. In contrast, the Committee treated the Japanese witnesses and their partisans with a degree of contempt and nonchalance. Instead, it was taken as given that the Japanese had not assimilated and placed the burden of proof on the Japanese to prove otherwise.

One of the reasons why the Japanese community had no analogue to the DiMaggios was the legacy of the systemic racism and exclusionary policies of nearly fifty years. The Japanese community in the West Coast had been the subject of multiple restrictions by state and local governments seeking to cut off its access to more lucrative forms of businesses. The typical Japanese-American was a small business-owner or truck-farmer not by choice, but often out of necessity. Exclusion and discrimination against Asian-Americans was an issue that enjoyed broad bipartisan support in West Coast politics. Various segregation laws were in place that treated these communities as aliens well before the 1924 Immigration Act (which was one of the more racist pieces of legislation passed by Congress) cut immigration from East Asia to a standstill. Japanese-Americans had the benefit of much more powerful nation-state patron in contrast to Chinese on the West Coast, but the restrictions meant that a good portion of Japanese-Americans were not recent immigrants, but natural-born citizens by 1941.

This is why the Japanese internment figures so largely within the curriculum of American history classrooms. The history of internment is not just one of flagrant racism mixed with poorly though-out EOs, but it also was the culmination of several long-term trends in American and West Coast history. It is very easy to draw a line between the initial wave of Asian immigration in the nineteenth century, larger patterns of discrimination and exclusion in the twentieth century that culminated in the internment in 1942. Wholesale internment did not come out of the ether, but rather American culture, society, and government were all primed for something like it to occur. The speed and totality of Japanese internment was a testament to these latent triggers in American society.

Aside from the subjective aspect, in which the behavior of the US is considered, there's also the objective aspect, as it was seen by the agent executing the actions, which can't be ignored to adequately analyse such behavior. From such point of view, the issue is how the loyalty of those ethnic groups in relation to the US and their motherland was observed by the authorities.

First, the internment was a reaction to a chain of well known events. A fact that was widely observed back then, even years before the internment camps, is that among the Axis countries Japan was the one constantly displaying more and more enmity towards the US, while the other two were focused on Europe. And then Japan outright attacked the US, in American soil, something that the other Axis nations would not dare (or were perceived as such). It was in this context, after the attack, that the decision to take the internment actions was taken.

Second, there were elements unknown to the public. The US secret service was privy to the fact that there were massive networks of spies from all those nations acting in the US and in other allied countries. Were there actual threats made by members of such groups against the State security? What did the secret service already knew, that could not yet be released to the public to justify the actions? How fanatic were supporters of the homeland regimes? I can share how the situation that was worrying the authorities in US has developed in Brazil, that had a much bigger Japanese community, and did not take actions of internment.

The Italian community in Brazil was sizable and perceived as integrated. Migrations began in mid-19th century, the culture was similar, intermarriage was common. Not so much with the younger 20th century Japanese community, that had a tendency to group itself in ethnic neighborhoods that other immigrants in Brazil did not had, to speak Japanese in their homes and in all their affairs, to have their own newspapers published only in Japanese while others were bilingual as required by law, and so on. Despite being published only in Japanese, Brazilian authorities knew that those newspapers used to trash the US and to celebrate Japanese expansionism, and after Brazil declared war on Germany (that was already attacking Brazilian property in Europe, without declaration), they started trashing Brazil. The Japanese community went on to turn more and more into itself, to the point that Japanese-Brazilian businessmen were being prohibited, under torture and death threats, to even conduct basic business with non-Japanese people.

Soon the most fanatic members of the community formed a nation-wide terrorist group by the name of Shindo Renmei. Non-Japanese people, Brazilians and immigrants alike, were frightened by their threats and acts of sabotage, but Japanese-Brazilians were even more frightened. While the tokkotai (the minions) walked with katanas to scare people, the leaders posed real threats against certain industries that were supplying the allied powers, such as the explosives industry and the parachutes industry. Overall their attacks managed to kill at least 23 people, almost all of them Japanese-Brazilians, and to injure about 150 people. Most of the killings happened after the allied nations had already won, to punish the makegumi (dirty hearts), those of the community that had the hearts so dirty that they dared to believe the news that Japan had surrendered and the war was over.

I hope this can provide the remaining elements for a more complete and fair analysis of the policies the US adopted, according to its point of view back then, facing the global war of which it was a defendant. More information on the developments in Brazil can be found on the very recommended book Corações Sujos (2000) by Fernando de Morais, which is my source for this post. There's also a 2012 movie by the same name, Corações Sujos (dirty hearts), very popular in Brazil.

Edit: grammar, specially Japanese words auto-correct can't detect