the world powers quickly increased. The Soviet Union and France began to rearm in the early thirties. In the Soviet Union, the mechanisation of the armed forces was part of a massive general industrialisation programme, the successive Five Years Plans, and the country soon had more tanks than the rest of the world combined, thousands of them being produced per year. In this period, before the rise to power of the Nazi Party in Germany, German officers were sent to observe and participate in development of armoured doctrine in the USSR. Red Army and German experts collaborated in developing the use of tanks based on second generation vehicles with turreted main weapons, and experimenting to design different chassis configurations and drive trains. One important acquisition for the Red Army turned out to be the purchase of a T3 chassis, using the Christie suspension, from US designer John Walter Christie, which served as the basis of the Soviet BT series of fast tanks. The Red Army tactics were influenced by the theoretical works of Marshal Mikhail Tukhachevsky who advocated "large scale tank warfare" as part of the deep battle doctrine. In France, the second largest tank producer, mechanisation was motivated by a need to compensate for severe manpower shortages due to a collapsed birth rate during World War I. This led to the development of a vast range of specialised armoured vehicles, not just tanks but also armoured cars, self-propelled guns, mechanised artillery, armoured tractors, armoured supply vehicles, armoured artillery observation vehicles, armoured command vehicles, half-tracks, and fully tracked armoured personnel carriers. As the mechanisation progressed, slowly the French armour doctrine began to reflect the increased capacity, evolving from direct infantry support, to independent breakthrough and eventually envelopment with the Infantry, and to deep strategic exploitation with the Cavalry. Despite the increase in tank numbers, in all countries financial constraints prohibited a full mechanisation of the entire armed ground forces. Necessarily, most of the divisions still consisted of infantry that was not even motorised. As a result, tanks tended to be allotted to special armoured units, where the limited and expensive expert maintenance and training capacity could be concentrated. Only the Soviet Union had enough tanks to equip an organic tank battalion in each infantry division. Nevertheless, France was the first to create large armoured units: in 1934 two Mechanised Corps were formed of 430 tanks each. In July 1935, in