On July 7, 1937, Japan launched a full-scale invasion of China, and China began a comprehensive War of Resistance. While a state of war fully existed between China and Japan, the National Government only issued a Statement of Self-Defense and War of Resistance but did not formally declare war on Japan. On December 7, 1941, Japan launched a surprise attack on Pearl Harbor, marking the start of the Pacific War. On December 9, 1941, the Government of the Republic of China issued its Declaration of War against Japan.
Full Text of the Republic of China’s Declaration of War Against Japan
, agreements, and contracts concerning the relationship between China and Japan are nullified. This is hereby proclaimed.
December 9, the 30th Year of the Republic of China (December 9, 1941 AD)
Chairman Lin Sen

The National Government’s Declaration of War Against Japan is recorded in the National Government Gazette, Vol. 30: Chongqing: 421 (ROC 30.12.10)