The early tanks were mechanically rudimentary. The thick armor generally prevented penetration by small arms fire and shell fragments. However, even a near miss from field artillery or an impact from a mortar HE round could easily disable or destroy the tank: if the fuel tank was ruptured, it could incinerate the tank's crew. A large caliber gun was recognized as a tactical necessity to attack machine gun positions and defeat any infantry field pieces found in the trench lines which could easily disable tank track with the HE ammunition. This was achieved by mounting a QF 6 pounder Hotchkiss light 57 mm naval gun mounted in the hull barbettes. Hull and track engineering was largely dictated by the terrain—the need to cross wide trenches—although the relationship between ground pressure and soil-vehicle mechanics was not resolved until the Second World War. Turrets were later introduced on medium and light tanks to react to ambushes during the advance.