This is 1945, before the decades long alliance and best friendship between US and Japan, before Internet, computers, and most electronics. Americans can't even read the signposts in the streets, let alone find and interpret the results of extremely secret operations.
Are you implying that in 1945 there were no people who spoke Japanese and English and Japan itself didn't have decades of emmigration to all parts of the world? The US committed one of its greatest crimes against humanity during the war against the huge number of Japanese Americans in the US.
He's referring to the US government rounding up US citizens of Japanese descent and moving them to camps during WWII. It was a horrific thing the US government did. Unfortunately, because humans can be horrible, I would disagree it was one of the greatest crimes against humanity in history.
