an independent Naval Air Arsenal was also established to streamline the testing and development of aircraft and weaponry. During their early years, these organizations were under the command of able air enthusiasts, who played a major role in the rapid expansion of Japanese naval aviation during the following decade. The London Naval Treaty of 1930 had imposed new limitations on warship construction, which caused the Navy General Staff to view naval aviation as a way to make up for the shortcomings in the surface fleet. In 1931, the air service pushed for and established the remainder of the 17 air squadrons that had been projected in the 1923 expansion plans. These were eventually combined into six air groups (kokutai) located at six bases around Japan. Furthermore, the Circle naval expansion programs featured an additional 12 air groups. They also included the development of specific aviation technologies and the acceleration of air crew training. The Circle One plan concentrated on developing new aircraft types, including large flying boats and land-based attack aircraft, as well as the building of seaborne units, both floatplanes and carrier aircraft. The Circle Two plan continued the buildup in naval aircraft and authorized the construction of two aircraft carriers. Shanghai incident (1932) In January 1932, clashes between Chinese and Japanese forces occurred in Shanghai. On 29 January, several aircraft from the seaplane tender Notoro, anchored in the Yangtze river, carried out low-level attacks on Chinese military positions in Zhabei, on artillery positions outside the city and on an armored train at a railway station in the northern part of the city. There were heavy civilian casualties and property losses, partly as a result of crude bombing techniques and mechanisms at the time. The Third Fleet consisting of the First Carrier Division with the carriers Kaga and Hōshō was also dispatched to the city. Kaga arrived off the entrance of the Yangtze River on 1 February, and was joined by Hōshō two days later. On board Hōshō were ten fighters and nine torpedo bombers, while Kaga had 16 fighters and 32 torpedo bombers. Altogether, the Japanese had eighty aircraft that could be deployed over Shanghai, mostly Nakajima A1N2 fighters and Mitsubishi B1M3 torpedo bombers. On 3 February, a number of the aircraft from the two carriers were deployed to Kunda Airfield, where they flew missions in support of Japanese ground forces. Aircraft from Hōshō participated in the IJN's