 By the time M4s reached combat in significant numbers, battlefield demands for infantry support and tank versus tank action far outnumbered the occasional opportunities of rear-echelon exploitation.United States doctrine held that the most critical anti-tank work - stopping massed enemy tank attacks - was primarily to be done by towed and self-propelled anti-tank guns, operated by "Tank destroyer" battalions, with friendly tanks being used in support if possible.Speed was essential in order to bring the tank destroyers from the rear to destroy incoming tanks.