and other regions dragged on near the end of 1937, the initial Chinese Air Force inventory of mostly American-made aircraft had become largely depleted, and the Soviet Union came to China's aid by supplying the bulk of aircraft to the Chinese Air Force for the next four years of war under the Sino-Soviet Treaty of 1937, and dispatching a Soviet Volunteer Group of aviators to assist with the combat operations in China; Chinese fighter pilots having mostly converted into the Polikarpov I-15 and I-16 series of fighter aircraft by early-1938. According to records from the USSR, 563 fighter planes and 322 bombers were provided to China by the Soviet Union, and these included the twin-engined Tupolev SB medium bombers and 4-engined Tupolev TB-3 heavy bombers, with the addition of Polikarpov I-153 fighters; the Chinese Air Force would remain with these increasingly obsolescent aircraft through 1941 as the Japanese made tremendous technological advancements to their combat aircraft designs and powerplant technology. Following the Japanese invasion of French Indochina, the United States took actions to assist the Republic of China against Imperial Japan, with the oil and scrap metal embargo and freezing of Japanese assets in the summer of 1941, and along with the Lend-Lease Act of which China was included as a beneficiary on 6 May 1941, the American Volunteer Group (a.k.a. "Flying Tigers") entered the war in December 1941 with the task of protecting the main Allied supply-line into China; the cross-Himalayan air-route known as "The Hump", between the Harbor of Rangoon in Burma, British bases in India, and the Chinese wartime port-of-entry in Kunming, China, and consequently, with the attack on Pearl Harbor, the U.S. officially joined the war against Japan. 1937 Battle of Shanghai/Nanjing At the outbreak of full-scale conflict of the Second Sino-Japanese War/World War II in August 1937, the Chinese Air Force had 645 combat aircraft, of which about 300 were fighter planes. Japan had 1,530 army and navy aircraft, of which about 400 were deployed in China. The frontline Chinese fighter squadrons were equipped primarily with the Curtiss Hawk IIIs and Hawk IIs, followed by Boeing P-26 Model 281 Peashooter, and Fiat CR.32. Bomber-attack aircraft consisted primarily of the Curtiss A-12 Shrikes, Douglas O-2s, Heinkel He-111, Martin B-10s and Northrop Gammas. The air war in China had become a testing-zone for the latest biplane fighter designs confronting the new generation of monoplane fighter and bomber