SARUFUTSU, Japan — More than a half-century has passed since the postmaster in this seaside hamlet on the frigid, northern tip of Japan pulled aside a young man and shared a secret. Somewhere in the village, the old man confided, was a lost graveyard hiding Korean bones.

It took years for Koichi Mizuguchi to grasp the significance of that utterance, and decades more to pry the grim truth from his tight-lipped neighbors: At least 80 Korean laborers died of abuse and malnutrition here as they built an airfield at the behest of the Japanese military during World War II. Eventually, Mr. Mizuguchi helped find the graves, and he and other residents began building a six-foot stone memorial at the site.

A decade ago, a village trying to preserve the memory of its wartime sins might have gone unnoticed in Japan. But Sarufutsu’s tiny village hall was inundated late last year with menacing phone calls denouncing residents as traitors. The campaign, orchestrated on the Internet, also called for a boycott of the village’s scallop industry. Shaken, the mayor ordered a halt to construction of the monument.

Coming to terms with its militarist past has never been easy for Japan, which tried to set aside the issues raised by the war as it rebuilt itself into the peaceful, prosperous nation it is today. But pressure to erase the darker episodes of its wartime history has intensified recently with the rise of a small, aggressive online movement seeking to intimidate those like Mr. Mizuguchi who believe the country must never forget.