From the long-distance perspective of an American, Asia looks like one of the world’s most peaceful places. And it is — for the moment. But when Vice President Joe Biden arrived in Tokyo on Monday, he stepped into what has suddenly become a dangerous diplomatic crisis between China and Japan. On the surface, it’s a dull dispute over a string of uninhabited Pacific islands. Underneath, as I realized on a recent visit to China, it’s a story reaching back some 75 years that involves war, brutality, rape and historical reckoning. And now threatens to drag in the U.S....

“It may be hard for you to understand,” an expert at Beijing’s Academy of Military Science told me in October, echoing several others to whom I spoke. “The nationalist feeling, the emotion toward Japan, is very strong.”...

Americans tend to think of World War II’s Pacific theater as a fight between the U.S. and Japan in places like Midway and Okinawa, lasting from 1942 to the atomic bombs of 1945. But for China, practically speaking, there was no “world” war — only a brutal invasion and occupation by Japan that began more than four years before Pearl Harbor. That fight cost 20 million Chinese lives, and included some of the worst atrocities of the entire war in the now infamous city of Nanjing, where up to 300,000 Chinese were massacred....