Having fought two wars against Japan (1894–95 and 1937–45), China's long-standing concern about the level of Japan's military strength surfaces periodically, and criticism of Japan's refusal to present a full version of the atrocities of World War II in its textbooks is a perennial issue. The relationship between China and Japan has been strained at times by Japan's refusal to acknowledge its wartime past to the satisfaction of China. Revisionist comments made by prominent Japanese officials and some Japanese history textbooks regarding the 1937 Nanjing Massacre have been a focus of particular controversy. Sino-Japanese relations warmed considerably after Shinzō Abe became the Prime Minister of Japan in September 2006, and a joint historical study conducted by China and Japan released a report in 2010 which pointed toward a new consensus on the issue of Japanese war crimes. However, in the early 2010s, relations cooled once more, with Japan accusing China of withholding its reserves of valuable rare earth elements.