Miki Dezaki, who first arrived in Japan on a teacher exchange program in 2007, wanted to learn about the nation that his parents had once called home. He taught English, explored the country and affectionately chronicled his cross-cultural adventures on social media, most recently on YouTube, where he gained a small following for videos like “Hitchhiking Okinawa” and the truly cringe-worthy “What Americans think of Japan.” One of them, on the experience of being gay in Japan, attracted 75,000 views and dozens of thoughtful comments.

Dezaki didn’t think the reaction to his latest video was going to be any different, but he was wrong. “If I should have anticipated something, I should have anticipated the netouyu,” he told me, referring to the informal army of young, hyper-nationalist Japanese Web users who tend to descend on any article — or person — they perceive as critical of Japan.

But before the netouyu put Dezaki in their crosshairs, sending him death threats and hounding his employers, previous employers and even the local politicians who oversee his employers, there was just a teacher and his students.

Dezaki began his final lesson with a 1970 TV documentary, Eye of the Storm, often taught in American schools for its bracingly honest exploration of how good-hearted people — in this case, young children participating in an experiment — can turn to racism. After the video ended, he asked his students to raise their hands if they thought racism existed in Japan. Almost none did. They all thought of it as a uniquely American problem.

Gently, Dezaki showed his students that, yes, there is also racism in Japan. He carefully avoided the most extreme and controversial cases — for example, Japan’s wartime enslavement of Korean and other Asian women for sex, which the country today doesn’t fully acknowledge — pointing instead to such slang terms as “bakachon camera.” The phrase, which translates as “idiot Korean camera,” is meant to refer to disposable cameras so easy to use that even an idiot or a Korean could do it.

He really got his students’ attention when he talked about discrimination between Japanese groups. People from Okinawa, where Dezaki happened to be teaching, are sometimes looked down upon by other Japanese, he pointed out, and in the past have been treated as second-class citizens. Isn’t that discrimination?

“The reaction was so positive,” he recalled. For many of them, the class was a sort of an a-ha moment. “These kids have heard the stories of their parents being discriminated against by the mainland Japanese. They know this stuff. But the funny thing is that they weren’t making the connection that that was discrimination.” From there, it was easier for the students to accept that other popular Japanese attitudes about race or class might be discriminatory.

The vice principal of the school said he wished more Japanese students could hear the lesson. Dezaki didn’t get a single complaint. No one accused him of being an enemy of Japan.

That changed a week ago. Dezaki had recorded his July classes and, last Thursday, posted a six-minute video in which he narrated an abbreviated version of the lesson. It opens with a disclaimer that would prove both prescient and, for his critics, vastly insufficient. “I know there’s a lot of racism in America, and I’m not saying that America is better than Japan or anything like that,” he says. Here’s the video:

Also on Thursday, Dezaki posted the video, titled “Racism in Japan,” to the popular link-sharing site Reddit under its Japan-focused subsection, where he often comments. By this Saturday, the netouyu had discovered the video.

“I recently made a video about Racism in Japan, and am currently getting bombarded with some pretty harsh, irrational comments from Japanese people who think I am purposefully attacking Japan,” Dezaki wrote in a new post on Reddit’s Japan section, also known as r/Japan. The critics, he wrote, were “flood[ing] the comments section with confusion and spin.” But angry Web comments would turn out to be the least of his problems.

The netouyu make their home at a Web site called ni channeru, otherwise known as ni chan, 2chan or 2ch. Americans familiar with the bottommost depths of the Internet might know 2chan’s English-language spin-off, 4chan, which, like the original, is a message board famous for its crude discussions, graphic images (don’t open either on your work computer) and penchant for mischief that can sometimes cross into illegality.

Some 2chan users, perhaps curious about how their country is perceived abroad, will occasionally translate Reddit’s r/Japan posts into Japanese. When the “Racism in Japan” video made it onto 2chan, outraged users flocked to the comments section on YouTube to attempt to discredit the video. They attacked Dezaki as “anti-Japanese” and fumed at him for warping Japanese schoolchildren with “misinformation.”

Inevitably, at least one death threat appeared. Though it was presumably idle, like most threats made anonymously over the Web, it rattled him. Still, it’s no surprise that the netouyu’s initial campaign, like just about every effort to change a real-life debate by flooding some Web comments sections, went nowhere. So they escalated.

A few of the outraged Japanese found some personal information about Dezaki, starting with his until-then-secret real name and building up to contact information for his Japanese employers. Given Dezaki’s social media trail, it probably wasn’t hard. They proliferated the information using a file-sharing service called SkyDrive, urging fellow netouyu to take their fight off the message boards and into Dezaki’s personal life.

By Monday, superiors at the school in Japan were e-mailing him, saying they were bombarded with complaints. Though the video was based almost entirely on a lecture that they had once praised, they asked him to pull it down.

“Some Japanese guys found out which school I used to work at and now, I am being pressured to take down the ‘Racism in Japan’ video,” Dezaki posted on Reddit. “I’m not really sure what to do at this point. I don’t want to take down the video because I don’t believe I did anything wrong, and I don’t believe in giving into bullies who try to censor every taboo topic in Japan. What do you guys think?”

He decided to keep the video online, but placed a message over the first few sentences that, in English and Japanese, announce his refusal to take it down.

But the outrage continued to mount, both online and in the real world. At one point, Dezaki says he was contacted by an official in Okinawa’s board of education, who warned that a member of Japan’s legislature might raise it on the floor of the National Diet, Japan’s lower house of parliament. Apparently, the netouyu may have succeeded in elevating the issue from a YouTube comments field to regional and perhaps even national Japanese politics.

“I knew there were going to be some Japanese upset with me, but I didn’t expect this magnitude of a problem,” Dezaki said. “I didn’t expect them to call my board of education. That said, I wasn’t surprised, though. You know what I mean? They’re insane people.”

Nationalism is not unique to Japan, but it is strong there, tinged with the insecurity of a once-powerful nation on the decline and with the humiliation of defeat and American occupation at the end of World War II. Japan’s national constitution, which declares the country’s commitment to pacifism and thus implicitly maintains its reliance on the United States, was in some ways pressed on the country by the American military government that ruled it for several years. The Americans, rather than Japan’s own excesses, make an easy culprit for the country’s lowered global status.

That history is still raw in Japan, where nationalism and resentment of perceived American control often go hand-in-hand. Dezaki is an American, and his video seems to have hit on the belief among many nationalists that the Americans still condescend to, and ultimately seek to control, their country.

“I fell in love with Japan; I love Japan,” Dezaki says, explaining why he made the video in the first place. “And I want to see Japan become a better place. Because I do see these potential problems with racism and discrimination.” His students at Okinawa seemed to benefit from the lesson, but a number of others don’t seem ready to hear it.