By 1926, German Army doctrine was all rewritten to fulfill this vision. Although at first the concept of the tank as a mobile weapon of war met with apathy, German industry was silently encouraged to look into tank design, while quiet cooperation was undertaken with the Soviet Union.[9] In late 1920s and early 1930s, Germans closely co-operated with Russians in the development of armored vehicles, which were tested at Kama tank school, near Kazan in the USSR. There was also minor military cooperation with Sweden, including the extraction of technical data that proved invaluable to early German tank design.[10]