In World War 2, Light, Medium and Heavy tank application to different roles was incorporated into doctrine. In the US, light tanks were expected to be used ahead of the main force, medium tanks to accompany the main thrust of attack, and by-their-nature slower heavy tanks being brought up to deal with any more significant opposition. In practice, US heavy tanks saw limited use due to the capacity limits of most dockyard equipment, preventing their delivery to the theatres of operation. This left a role-based classification, the tank destroyer, to evolve from the need to move artillery pieces and set ambushes for axis tanks.