M1A1 USA Light tank

The M1 Combat Car, officially Light Tank, M1, was a light tank used by the US Cavalry in the late 1930s  and developed at the same time as the infantry's very similar M2 light tank. After the Spanish Civil War, most armies, including the US Army, realized that they needed tanks armed with cannons, not merely vehicles armed with machine guns, and so the M1 became obsolete.

                   
M1A1 light tank of an unidentified training unit, 1941. This variant (17 built in 1937) received a new octagonal turret. 

The hull was 40 cm longer (to 4.44 m – 17 ft 7 in) and the two bogies were farther apart. The next M1A1E1 (7 produced) received a new Guiberson diesel engine. 
                   
They led to the development of the M2 light tank.

Battle of France 1940 cause the reasons assembly to 37 mm cannon
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The National Defense Act of 1920 set tanks as the responsibility of the infantry and the general staff defined the purpose of tanks as the support of infantry units. Light tanks were defined as weighing five tons or less – so they could be carried by trucks – and medium tanks no greater than 15 tons to meet bridge weight limits. With very tight restrictions on spending, tank development in the US was limited to a couple of test vehicles a year. 

                      
Civil War veterans (wearing Grand Army of the Republic uniforms) inspect an M1 Combat Car at the 1939 World's Fair in New York
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The mechanization of the army was promoted by General Douglas MacArthur (Chief of Staff of the US Army) who believed that the cavalry should have tanks for an exploiting role rather than acting in support of the infantry. To allow US Army cavalry units to be equipped with armored fighting vehicles, the tanks developed for the cavalry were designated "combat cars". 

In the mid-1930s, the Rock Island Arsenal built three experimental T2 light tanks inspired by the British Vickers 6-ton tank. At the same time, they built a light tank similar to the T2 for the cavalry – the T5 combat car. The only major difference between the two was that the T5 used vertical volute suspension while the T2 had leaf springs as on the Vickers. The T5 was developed further and the T5E2 was accepted for production as the "M1 combat car". 

The M1 entered service in 1937. A change to the suspension so that the idler wheel rested on the ground ("trailing") increased the length of track in contact with the ground and improved the ride. Together with a different engine and improved turret, this produced the M2 combat car.  

In 1940, the distinction between infantry and cavalry tank units disappeared with the establishment of the armored force to manage all tanks in the US Army. The "combat car" name was superfluous, and the cavalry unit tanks redesignated the M1 combat car as the "light tank M1A1" and the M2 combat car as the "light tank M1A2". 

The M1 and M2 combat cars were not used in combat by the US Army during World War II; though some were used for training purposes.