The ongoing war in Europe at the time proved that, in order for a vehicle to be successful, it needed a proper cannon. In October 1939, despite its potential, the Mechanized Cavalry Board recommended further development and test of the T7 Combat Car program, and all other such convertible vehicles, be canceled, marking the end of wheel-tracked combat vehicle development for the U.S. Army. Interwar M1 Combat Car and M2 Light Tank The M1 Combat Car was a tankette that entered use with the U.S. Cavalry in the late 1930s. Under the terms of the Defense Act of 1920, tanks were restricted to infantry units, which were equipped with M1 tanks armed with twin machine gun turrets by 1935. To allow U.S. Army cavalry units to be equipped with armored fighting vehicles, the tanks developed for the cavalry were designated "combat cars". The Cavalry branch opted for a single, larger turret to be mounted on their M1s. Other upgrades included improved suspension, improved transmission, and better engine cooling. The Combat Car designation was dropped in 1940, and the vehicle was renamed as the light tank M1A2. The next tank to be designed, the M2 Light Tank, was developed in 1935 by Rock Island Arsenal for the infantry branch of the U.S. Army. The design originated from the earlier T1 and 1929 T2 Medium Tank designs, and drew limited inspiration from the British Vickers 6-ton. Its main weapon was one .50 BMG M2 Browning machine gun, and one .30-06 M1919 Browning machine gun, both installed in a small, one-man turret. After only 10 units were delivered, the Infantry branch decided to switch to a twin turret configuration, with the M1919 moved to a second turret. These early twin-turret tanks were nicknamed "Mae West" by the troops, after the popular movie star. The twin-turret layout was inefficient, but was a common feature of 1930s light tanks derived from the Vickers, such as the Soviet T-26 and Polish 7TP. An improved version of the M2 Light Tank, the M2A4, armed with a high-velocity 37 mm gun M3 and coaxial machine gun in a single turret, served with the Marines' 1st Tank Battalion on Guadalcanal in 1942–43. The M2A4 was the only model of the M2 line to see combat. The Rock Island Arsenal also started work on a new medium tank, based on the design of the M2 Light Tank. Initially designated the