German manufacturing of tanks began in 1934, in violation of the Treaty of Versailles. German industry had not produced arms in many years, and time was required to develop the necessary techniques for hardening steel and for the development of tooling to produce arms. The first tank type produced was the two-man, air-cooled Pz I light tank, designed and produced by the Krupp works as a training vehicle. It was not designed for use in combat; instead, the main battle tank of the German army was to be the Panzer III but delays in its development and manufacture led to the production of an interim vehicle, the liquid-cooled Panzer II, which began production the following year. It was not clear yet how tanks would be used in the next war, nor was it apparent that tank-versus-tank combat would be important; some military theorists envisioned tanks operating in support of infantry. The cavalry arm of the Heer saw the major function of tanks to be that of reconnaissance. This was in marked conflict with the ideas of the armoured warfare proponents of Germany, chiefly Heinz Guderian, Wilhelm Ritter von Thoma, and Walther von Reichenau. It was not until 1937–38 that the first Panzer III and Panzer IV medium tanks began to appear.