An aspiring history major has a question about Japanese internment.

So, you want to know if internment was "justified". Like any historical question, it depends on what you means by "justified".

Lets define justification along these lines: were the concerns regarding Japanese Americans as a potential "fifth column" justified? To ascertain this, we need to understand who Japanese Americans were and look at some of the internal debate regarding the "Japanese problem".

First, it is important to note that by 1941, the vast majority of Japanese in the United States were second or third generation, born on US soil. This is because of the "gentlemen's agreement" between the US and Japan in 1907 that effectively stopped Japanese migration to the United States (though not entirely, hence the 1924 National Origins Act). They had little knowledge of Japan, and Imperial Japan were themselves suspicious of them and the possibility of duplicity. As a result, the vast majority of seditious activity undertaken by the Japanese government were carried out through white and black Americans. So in terms of the actual strategy of the Japanese government, the imagined "fifth column" of Japanese Americans was precisely that, imagined.

Now, perhaps the government was misguided or misinformed? Well, under the order of FDR, Curtis Munson was tasked to gather information about the loyalty of the Japanese in November of 1941, prior to Pearl Harbour. Some of these excerpts tell us that, no, internment was not necessary. I will now quote extensively from the Munson Report:

There are still Japanese in the United States who will tie dynamite around their waist and make a human bomb out of themselves. We grant this, but today they are few. Many things indicate that very many joints in the Japanese set-up show age, and many elements are not what they used to be. The weakest from a Japanese standpoint are the Nisei. They are universally estimated from 90 to 98 percent loyal to the United States if the Japanese-educated element of the Kibei is excluded. The Nisei are pathetically eager to show this loyalty. They are not Japanese in culture. They are foreigners to Japan. Though American citizens they are not accepted by Americans, largely because they look differently and can be easily recognized...

they are inclined to enlist before being drafted. The Army is extremely high in its praise of them as recruits... They are beginning to feel that they are going to get a square deal and some of them are really almost pathetically exuberant….The story was all the same. There is no Japanese `problem' on the Coast. There will be no armed uprising of Japanese. There will undoubtedly be some sabotage financed by Japan and executed largely by imported agents... In each Naval District there are about 250 to 300 suspects under surveillance. It is easy to get on the suspect list, merely a speech in favor of Japan at some banquet being sufficient to land one there. The Intelligence Services are generous with the title of suspect and are taking no chances.

The Munson Report makes clear: there was no mass threat of Japanese sabotage. Certainly no more than German or Italian Americans. Certainly, some German and Italians were interned, but on an infinitely smaller scale (for instance, of the over 1 million Germans in the US at the time only 11 thousand or so were interned). So, the question is, why Japanese?

Certainly, as you point to, Pearl Harbour acted as a catalytic event in spurring mass internment. But it certainly wasn't the beginning. As early as 1936 the FDR administration was compiling list of potential internees in Hawaii. So, yes, while Pearl Harbour is central to understanding the hysteria around internment, it doesn't fully explain its depth and scale.

The answer is race. The decades preceding the war were defined by "yellow peril", particularly in the West Coast of North America. Asians of all stripes were racialized as deviant, criminal, untrustworthy, and unable to assimilate to the United States. Sustained labour and political campaigns worked to erect restrictive migration laws. The Webb-Haney Act restricted the ability of Japanese to purchase land in California for example (and similar acts were passed in Oregon and Washington as well). Japanese anxieties were largely imaginary, however Pearl Harbour crystallized and punctuated long existing racial stereotypes of people who looked Japanese but were Americans. But these anxieties were not born out in the opinion of those embedded within these communities or the actual strategies carried out by Imperial Japan.