The Chinese city of Nanjing has suspended its sister-city relationship with Nagoya, Japan, after Nagoya’s mayor expressed doubts that the Japanese Army’s 1937 Nanjing Massacre actually took place, the Nagoya City Hall said Wednesday.

The falling out began Monday, when Nagoya’s mayor, Takashi Kawamura, told a visiting delegation of Chinese Communist Party officials from Nanjing that he doubted that Japanese troops had massacred Chinese civilians. Most historians say that at a minimum, tens of thousands of civilians were slaughtered in Nanjing in one of the most infamous atrocities of Japan’s military expansion across Asia in the early 20th century.

The falling out underscored how differing views of history remain a problem in Japan’s ties with the nations that it once conquered. While such denials are common by Japanese conservatives like Mr. Kawamura, they are rarely raised in such a public manner, or directly to Chinese officials. But there is also a widely shared perception in Japan that China’s government plays up the massacre for its own propaganda purposes....