“Don’t get me wrong,” said Mr. Hasegawa, a fisherman. “I don’t think that the bombing of Hiroshima was a good thing.” Staring at the furious grey channel where the Pacific Ocean meets the Sea of Okhotsk off Hokkaido in northern Japan on a cold, clear day last March, he spoke like a trauma victim reliving the past: “But if the Americans had dropped the atomic bomb a month earlier, those islands out there would still be Japan’s.”

Were I unaware of the chronology of the summer of 1945 and had we been anywhere else, such a comment would make the engaging 64-year-old seem insensitive or odd. Yet on the horizon three miles in the distance were the snow-covered banks of one of Russia’s Kuril Islands, known to the Japanese who lived there until 1945 as Suishojima of the Habomai group. Mr. Hasegawa’s father was among the 17,291 Japanese who called it and several other nearby islands home.* Admiral Yamamoto gathered his fleet there in 1941 to attack Pearl Harbor, and the region was once one of the three richest fishing grounds in the world, replete with salmon, herring, and cod.

In August 1945 wartime Emperor Hirohito announced Japan’s cataclysmic losses following America’s nuclear decimation of Hiroshima and Nagasaki, its firebombing of most other cities, and its devastation of Okinawa island in the East China Sea. Equally important, Russia had disavowed its neutrality pact with Japan, and Soviet troops were advancing into Japanese-controlled Manchuria, northern Korea, and a number of islands around Hokkaido. As the emperor told his defeated subjects with staggering understatement, “The war situation has developed not necessarily to Japan’s advantage.”

What parts of its massive empire Japan would forfeit were then unknown. In the coming years, an area that once resembled an enormous octopus spanning North China and the southern Pacific near Australia would be reduced to the seahorse-shaped nation that we are now familiar with. But this reality has yet to be accepted fully in Japan, especially among people like Mr. Hasegawa, whose lives were upended by history. They were left to imagine any number of alternate realities.

On September 2, 1945, Hirohito’s representatives signed surrender papers to American officers aboard the USS Missouri. At the same moment, Soviet soldiers overwhelmed the islands that the Japanese continue to call the Northern Territories (yet which are known internationally as the southern part of Russia’s Kuril Island chain). At the Yalta Conference in February 1945, Franklin Roosevelt promised these islands to Joseph Stalin in exchange for his troops’ entry into the war on the side of Allies. Within three days of the soldiers’ arrival on the southern Kurils, the Russians began to deport most of the Japanese to Hokkaido, although some were also taken to POW camps in Siberia. 20,000 Russians live on these islands today, and that, to paraphrase Vladimir Putin’s current mood, would appear to be that. Except, of course, for the evicted islanders and their descendants....