The Second Sino-Japanese War (July 7, 1937 – September 9, 1945), so named due to the First Sino-Japanese War of 1894–95, was a military conflict fought primarily between the Republic of China and the Empire of Japan from 1937 to 1945.ithin a few days of the attack on Pearl Harbor, China joined the Allies and formally declared war against Japan, Germany and Italy. As the Western Allies entered the war against Japan, the Sino-Japanese war would become part of a greater conflict, the Pacific theatre of World War II.