In the mid-1930s, the British Army was redefining its approach to armoured warfare. By 1936, the War Office had split tank development into two categories: infantry tanks, heavily armoured to support troops in direct assaults, and cruiser tanks, fast and mobile, designed to exploit breakthroughs and strike deep into enemy territory. Out of this doctrine came the Tank, Cruiser, Mk I (A9), the first true British cruiser tank. The origins of the A9 lay in 1934, when Sir John Carden of Vickers-Armstrong was asked to design a cheaper replacement for the ageing Medium Mark II. The result, completed as the A9E1 pilot model in 1936, incorporated ideas from the cancelled Medium Mk III but kept costs down by using commercial components, including a bus engine. It was the first British tank with a fully powered turret traverse system, developed by Nash & Thompson, the same firm supplying the Vickers Wellington bomber. Though its armour was light—no more than 14 mm thick—the A9 could reach 25 mph and mounted the new high-velocity 2-pounder gun, supported by a coaxial machine gun and two additional machine-gun turrets beside the driver. This unusual arrangement brought the crew up to six. Tests showed flaws in protection and mechanical reliability, but the A9 was accepted in 1937 as an interim cruiser until more advanced Christie-suspension designs were ready. Orders went to Harland & Wolff and Vickers, with 125 eventually produced. Its chassis later provided the basis for the Valentine infantry tank, which would go on to serve far longer in the war. The first Cruiser Mk Is reached units in early 1939. Only 36 were in service when the war began, with more arriving through 1940. The tank saw its first real action in France and later in Greece and North Africa. Against early Italian armour and the lighter German Panzers, the 2-pounder gun proved effective, and the A9 played a role in Britain’s initial desert campaigns. But as the Germans introduced heavier Panzer IVs in 1941, the A9’s weaknesses became clear. Its thin armour was highly vulnerable, especially around the machine-gun turrets, and its gun lacked high-explosive ammunition. Mechanical reliability was another problem, with frequent track failures hampering operations. By 1941, the A9 was already outdated, but as Britain’s first cruiser tank it marked a turning point. It embodied the new doctrine of speed and mobility over heavy armour, paving the way for later cruiser designs that would dominate British tank development throughout the war.