(HEAT) shells fired by other tanks is of primary importance, but tank armour also aims to protect against infantry mortars, grenades, rocket-propelled grenades, anti-tank guided missiles, anti-tank mines, anti-tank rifles, bombs, direct artillery hits, and (less often) nuclear, biological and chemical threats, any of which could disable or destroy a tank or its crew. Steel armour plate was the earliest type of armour. The Germans pioneered the use of face hardened steel during World War II and the Soviets also achieved improved protection with sloped armour technology. World War II developments led to the obsolescence of homogeneous steel armour with the development of shaped-charge warheads, exemplified by the Panzerfaust and bazooka infantry-carried weapons which were effective, despite some early success with spaced armour. Magnetic mines led to the development of anti-magnetic paste and paint. From WWII to the modern era, troops have added improvised armour to tanks while in combat settings, such as sandbags or pieces of old armour plating. British tank researchers took the next step with the development of Chobham armour, or more generally composite armour, incorporating ceramics and plastics in a resin matrix between steel plates, which provided good protection against HEAT weapons. High-explosive squash head warheads led to anti-spall armour linings, and kinetic energy penetrators led to the inclusion of exotic materials like a matrix of depleted uranium into a composite armour configuration. Reactive armour consists of small explosive-filled metal boxes that detonate when hit by the metallic jet projected by an exploding HEAT warhead, causing their metal plates to disrupt it. Tandem warheads defeat reactive armour by causing the armour to detonate prematurely. Modern reactive armour protects itself from Tandem warheads by having a thicker front metal plate to prevent the precursor charge from detonating the explosive in the reactive armour. Reactive armours can also reduce the penetrative abilities of kinetic energy penetrators by deforming the penetrator with the metal plates on the Reactive armour, thereby reducing its effectiveness against the main armour of the tank. Active protection system The latest generation of protective measures for tanks are active protection systems. The term "active" is used to contrast these approaches with the armour used as the primary protective approach in earlier tanks. Soft kill measures, such as the Russian Shtora countermeasure system, provide protection by interfering with enemy targeting and fire-control systems, thus making it harder for the enemy threats to lock onto the targeted