TLDR: The first thing that happened in WWII for the United States was the attack on Pearl Harbor. The second thing is a Japanese pilot crash-landed on Ni'ihau on his way home from bombing Pearl, and a Japanese immigrant and a couple Japanese-Americans decided to take his side and help him (treason, of course). In the end a couple people were shot.
Edit: I'll add some discussion from below to the top-level comment, I think it improves it.
In a situation where you have no data (hostilities just started), this incident being one of the first things that happen is, in fact, quite striking. And, critically, these people weren't spies or saboteurs. They were rando civilians who, without premeditation or contact with enemy government, decided on the spot to help the enemy. If you were a commander in that war you would rightly find it terrifying. It is, we might say, a bad look.
I bring it up in response to the story because it's an obvious part of the narrative about Japanese internment camps in the U.S. in WWII that is suppressed because it is inconvenient and uncomfortable. That doesn't mean we should ignore it, though.
Obviously, this event makes the actions the U.S. government took look more justifiable. The fact that it happened so quickly after hostilities started is also striking. The idea that Japanese immigrants and Japanese-Americans are more likely to turn against their adopted country than, say, German or Italian immigrants seems more plausible in this light, and that helps understand the decisions made by people in power (in addition to stuff you already know, like Japanese is a very different language to English and culture to American, far more different than either German or Italian, and Japanese people and other East Asians are much more easily visually distinguished, etc.).
And, I will reiterate, in my mind it doesn't justify interning tens of thousands of citizens on the basis of ethnicity. It seems obviously wrong to me, either you're a citizen and you get the same rights as everyone else, or you're not. Discriminating on the basis of ethnicity is...racism? Not quite, but I suppose that's the word people use. It is a stain on the country's history.