22 before he was killed (he was posthumously raised to the rank of colonel in the Chinese Air Service). During the Second Sino-Japanese War (1937–1945), the ROCAF participated in attacks on Japanese warships on the eastern front and along the Yangtze river including support for the Battle of Shanghai in 1937. The Chinese frontline fighter aircraft were initially mainly the Curtiss Hawk IIs and Hawk IIIs (including those license-built locally at the CAMCO plant) and the Boeing P-26 model 281, and engaged Japanese fighters in many major air battles beginning on August 14, 1937, when Imperial Japanese Navy warplanes raided Chienchiao airbase; "814" has thus become known as "Air Force Day". Chinese Boeing P-26/281 fighters engaged Japanese Mitsubishi A5M fighters in what is among the world's first aerial dogfighting between all-metal monoplane fighter aircraft. A unique mission in May 1938 saw two Chinese B-10 bombers fly a mission over Japan, but dropping only propaganda leaflets over the Japanese cities of Nagasaki and Saga, four years before 1942 Doolittle Raid on Japanese home islands. It was a war of attrition for the Chinese pilots, as many of their most experienced ace fighter pilots, such as Lieutenant and Colonel Kao Chih-hang were lost early in the war. Code breaking operations played a role in the conflict. A Japanese radio intercept unit was attached to the landing forces at Shanghai. IJN Lt. Commander Tsunezo Wachi and a Lt. Yamada, an expert in Chinese codes, enabled Japanese to make preemptive strikes against ROCAF airfields. As the War of Resistance raged on after the Fall of Shanghai, Nanking and Taiyuan at the end of 1937, new main battle lines were immediately drawn at the Battle of Wuhan, Battle of Taierzhuang, Battle of Canton, and the ROCAF received its primary support through the Sino-Soviet Non-Aggression Pact of 1937, along with the support of the Soviet Volunteer Group, from 1937 to 1939 in return for raw materials. Following the Japanese invasion of French Indochina, the U.S. enacted an oil and steel embargo against Japan in August 1941, and China was included in the Lend-Lease Act which superseded the Sino-Soviet Non-Aggression Pact of 1937. As the Battle of Chonqing-Chengdu continued into 1941 with Operating 102, the Japanese began preparations for Operation Z; the attack plan on Pearl Harbor. In the latter half of the Sino-Japanese War. World War II, the ROCAF was augmented by a volunteer group of