After the Russians relinquished claims to Manchuria in 1906, the Japanese declared it a jointly controlled territory (jointly controlled with China) under the terms of the Russian-Japanese peace treaty. Now though the Qing Chinese Empire wanted its land returned as the threat of attack from Russia had disappeared following the peace treaty. However the Japanese wanted to keep Manchuria as they had uncovered large mineral deposits needed to help the Japanese economy and improve the Japanese metallurgy business. This isn't seen as a good enough reason to give Manchuria to Japan, and Qing China declares war against Japan on March 23rd 1907, in order to protect China's sovereignty and to retake Chinese land, starting the second Sino-Japanese war (however it was still included in the World War