Battle tanks (of Heavy, Medium and Light types), Torpedo tanks (utilising large trench mortars), Engineer tanks, Supply tanks and Ambulance tanks During the inter-war years, British tank doctrine evolved through experimental trials and the works of J.F.C. Fuller, P.C.S. Hobart and B.H. Liddell-Hart. By 1936, these settled on the roles of light tanks for reconnaissance, infantry tanks to support an advance, and cruiser tanks in the cavalry role, using mobility to exploit situational advantages. The works were further explored by Heinz Guderian in the development of German tank doctrine and Blitzkrieg for the opening stages of World War II. Other nations continued to use the light, medium and heavy designations. US and Soviet forces also incorporated the tank destroyer concept, allowing their light, medium, and heavy tanks to prioritise works with the infantry. Soviet and US forces added the concept of the flame tank, armed with a flamethrower. During the course of the war, German forces added command tanks, specialised to the task of co-ordinating tank formations. This idea caught on with other nations. Development of British doctrine added howitzer-armed close support tanks, similar to the older torpedo tank role. These soon became critical to launching smoke, and post-war smoke dischargers became common on tanks. Both command and close support tanks were typically based on the type of tank they were supporting, so may not be considered a completely separate classification. With the fall of France, the need for infantry tanks to advance with troops started to be replaced with a need for Assault tanks, a new class with heavier frontal armour to take on battlefield defences. Infantry tanks proved capable in this new role however, and the designation was rarely applied outside of experimental production. The term saw limited use with both British and US forces in joint development. Hobart would later return to Martel's idea of Engineer tanks in the 1944 run-up to D-Day with Hobarts Funnies, and specialised tanks became a core component of the modern battlefield. Towards the end of the war, increases in tank engine power started to create the possibility of multi-role vehicles. British light tanks had largely been replaced with armoured cars and carriers, and engineers proposed a new Universal tank coupling Cruiser tank mobility with Infantry tank armour. The concept became redundant when Cruiser tank armour increased anyway, rendering the infantry tank obsolete. Post-war the light, medium, and heavy designations remained prevalent