France the 4th Cavalry Division was transformed into the 1e Division Légère Mécanique, the first French armoured division of the Cavalry. In Germany, after the Nazi Regime started open rearmament in March 1935, on 15 October 1935 three Panzerdivisionen were formed. Though some tank brigades were part of the Cavalry or Infantry arm, most German tanks were concentrated into a special branch, from 1936 called the Panzerwaffe. The precise interpretation of this phenomenon has proven controversial among military historians. Traditionally, it has been seen as part of a "Blitzkrieg strategy" of swift world conquest by means of armoured forces. Later it has been argued, among others by Karl-Heinz Frieser, that the German army in the 1930s did not even possess an explicit Blitzkrieg tactical doctrine, let alone strategy. This would have been reflected by the relatively unimpressive rate of tank production and development. During the 1930s the United Kingdom gave priority to the Royal Air Force and Royal Navy. The British Army began the conversion of its cavalry from horse to tanks and all but a few regiments were fully converted by 1939. The British 1st Armoured Division was formed, as the "Mobile Division", in November 1937. Before the Second World War actual use of armoured fighting vehicles was limited. Both sides used Italian, German and Soviet tanks during the Spanish Civil War but these proved to be vulnerable to antitank guns due to their thin armour. Traditionalist elements within the Red Army used this to diminish the influence of proponents of mechanisation. Tukhachevsky himself was executed in 1937. Nevertheless, during the Soviet-Japanese Border Wars of 1938 and 1939, the Soviet forces tested modern armoured warfare tactics. General Georgy Zhukov in the summer of 1939 combined mass tank manoeuvres with artillery and air attacks, to defeat the Japanese Imperial Army at the Battles of Khalkhin Gol at Nomonhan in Mongolia. Partly as a result of the experiences in Spain, the Soviet Union began the development of a new generation of medium and heavy tanks, sporting much stronger armour and armament. World War II Poland In their Invasion of Poland during September 1939, German forces applied a narrow cooperation between large armoured units – of the Panzerwaffe and the Cavalry – and "active" infantry divisions to break the Polish defensive lines and pursue the defeated enemy forces. The more limited and dispersed Polish armoured units were quickly destroyed. The Red Army,