Thousands of mourners gather here in the eastern Chinese city of Nanjing on Thursday. They're here to mark the anniversary of a notorious massacre by Japanese forces 75 years ago. China says Japanese troops killed 300,000 people in what was then the national capital. A post-war Allied tribunal put the death toll at 142,000, but some conservative Japanese politicians and scholars still deny a massacre even took place. And the refusal of some in Tokyo to admit to wartime atrocities has put a strain on Sino-Japanese relations. A wave of anti-Japan protests broke out across China in 2005, when some Japanese high school textbooks omitted references to a number of atrocities committed during Japan's occupation of northern China. This September China saw renewed protests after Japan's purchase of islands to which Beijing also lays claim. Japanese-Chinese relations formally resumed in 1972 after decades of diplomatic silence.