survive all existing anti-tank weapons and be cheap enough for mass production in peacetime. The next year, Vickers had a two-man tank design, with a machine-gun and powered by a civilian Ford V8 engine of . The prototype of October 1936 weighted had a maximum speed of only but carried of armour and was mechanically reliable. The A11, Infantry Tank Mk I, was the first Infantry tank (I tank) and the first practical expression of the decision to split design into I tanks and cruiser tanks, with different functions and tactics, supplied to separate units and formations. The 1935 edition of the War Office publication, Field Service Regulations (FSR), containing the principles by which the army was to act to achieve objectives, was written by Major-General Archibald Wavell, made breakthrough the responsibility of infantry divisions with the support of Army Tank Battalions, equipped with specialised vehicles for infantry-artillery co-operation, the slow and heavy Infantry tanks. Once a breakthrough had been created, a Mobile Division containing a tank brigade with light and cruiser tanks, would advance through the gap and use the speed and range of its tanks to surprise the defender and attack flanks, headquarters and non-combatant units. By 1939, further amendments to FSR added counter-attacks on an enemy armoured breakthrough. (The codification of the difference between Infantry and cruiser tanks and their functions in FSR 1935, accidentally created an obstacle to all-arms co-operation that lasted long into the Second World War.) Defence against tanks could be achieved by troops finding physical obstacles and by controlling their own anti-tank guns. The obstacles could be woods and rivers or minefields as long as they were covered by fire from other weapons. In places lacking convenient terrain features, lines-of-communication troops would also need anti-tank guns and be trained to set up localities suitable for all-round defence The need for economy in the design and production of the A11, which was too small for a radio, led to work on a successor, the A12, Infantry Tank Mk II in 1936. Capable of , the A12 was still slow but had of armour, making it almost invulnerable to tank guns and standard foreign guns like the German 37 mm Pak 36 anti-tank gun. The tank had a four-man crew and a turret big enough for a radio and an Ordnance QF 2-pounder high-velocity gun, firing solid projectiles capable of penetrating all 1939–1940 German tanks.