South Korean newspapers splashed a photo of Japanese Prime Minister Shinzo Abe in a military jet trainer on their front pages on Thursday, saying it was a reminder of Japan's colonial-era atrocities. (Jung Yeon-jejung/AFP/Getty Images)

For several years during World War II, a branch of the Japanese Imperial Army known as Unit 731 operated a vast biological and chemical research program in the Chinese province of Manchuria. The Japanese researchers conducted blood-curdling experiments on Chinese prisoners, many of them civilians, injecting them with chemicals, mutilating them, removing organs and committing other unspeakable acts that ultimately killed 3,000 and sickened 300,000 more. The unit number, 731, has since become synonymous with some of Japan's cruelest wartime abuses in Asia.

Those abuses, like the war itself and even the imperial government that launched it, are well in the past. Strangely, though, it is Japan itself that has been resurfacing them lately. A number of right-leaning politicians, including Prime Minister Shinzo Abe, who has waffled between issuing German-style apologies for the abuses and suggesting that maybe they weren't so bad, have been bringing them up. Even in interviews with the Western press, Abe continues to raise the bizarre question of whether Japan's invasions of several Asian neighbors counted as "aggression." When three high-level Japanese politicians visited a controversial Tokyo shrine honoring the country's war dead, including a number of war criminals, they carried a token from Abe. He has also previously argued that the Asian "comfort women" forced into sexual slavery by Japanese troops were actually prostitutes, an argument other Japanese politicians still make.

The most recent provocation came Sunday. Abe, visiting a Japanese air force base, climbed into a T-4 training jet for a photo op. He stuck up his thumb and smiled as cameras clicked away. Emblazoned across the side of the jet, right beneath the prime minister's smiling face, was the number "731."

While it's still not clear whether or not this was deliberate -- it's entirely possible the incident was an honest mistake, as Japanese officials have suggested -- it has further infuriated Japan's former victims and, rightly or wrongly, deepened the perception that many Japanese leaders are at best unrepentant about wartime abuses and at worst proud of their country's imperial past. Outraged South Korean and Chinese officials condemned Abe's behavior, with South Korea's largest newspapers running the offending photo on their front pages.

Japan-watchers in Asia and elsewhere are debating whether the photo was an unfortunate gaffe or a intentional insult to Unit 731's victims. In the absence of hard proof, it's probably safest to assume innocence. Either way, though, there's no denying the rising trend of nationalist chest-thumping by some Japanese leaders, who seem to increasingly hint that imperial Japan was not so bad and that maybe the country's pacifist constitution, imposed by the United States after the war, no longer fits.

I asked Jennifer Lind, a Dartmouth professor who studies Northeast Asia and is a consistently fair-minded observer of the region, what to make of the 731 incident. Here's what she said in an e-mail response (I've added a link to the Hashimoto incident she mentions):

This episode just underscores to Japan's neighbors, and to the rest of the world, that Japan's people and government are not aware of the terrible atrocities that they committed against their neighbors in the past.

So even if this was an inadvertent error, the fact that the Japanese military and government could make a gaffe like this shows such a clear lack of awareness and lack of sensitivity about Japan's past atrocities, and about the feelings of Japan's neighbors and past victims.

Japan really reached a low point with this episode -- it was discussed by a prominent, up-and-coming Japanese politician, Toru Hashimoto, who (referencing the wartime sex slave program) explained that we should understand that stressed-out guys need to rape women [as occurred during the wartime "comfort women" abuses]. I am beyond disgusted, and hope that the good people of Japan are too.

This is a good opportunity to note that many people in Japan, including some politicians, do in fact oppose this sort of behavior and are saying as much. Japan is a populous, vibrant country, and it can't be defined by the activities of a few nationalists with a habit of offending the rest of Asia. Still, perceptions matter, and those nationalists are playing a disproportionate role in shaping perceptions of Japan and its foreign policy abroad.