| Reactive armor is another step in the long-standing arms race between armor and munitions designers. When the anti-tank missile became common (around the time of the Six-Day War) there was a time when armor technology had lagged behind the capabilities of the shaped charge warheads found in such missiles and even in tank gun rounds (see HEAT). Since the penetration ability of these weapons is dependent on the charge retaining its precise shape and detonating at a specific range from the surface of the armor, it was reasoned that disrupting the charge itself before it could detonate would result in the armor being able to 'turn' the blast. So designers began to cover tank armor with thin plates of prefragmented steel, beneath which were flat sheets of high explosive. When the long fusing probe on the forward end of an incoming anti-tank missile struck the tank, or alternatively when proximity fuses on the armor detected fast-moving incoming objects, the sheet of explosive in the track of the incoming projectile would explode, sending a cloud of shrapnel and hot gas down the path of the incoming warhead. Sometimes the shrapnel would destroy the warhead, or at least disrupt the shape of it enough to nullify the shaped charge's penetrating power; but even if the charge was 'wobbled' slightly, the resultant angle at which the detonation hit the armor would often provide it with enough extra 'thickness' to prevent penetration.
This invention, needless to say, has not been popular with infantry personnel tasked with staying with the tanks. In Grozny, Chechnya the Russian army learned the hard way that if you have only rudimentary sensors on your armor and you send your tanks into city streets, someone leaning out a top floor window to drop a rock on the tank will detonate the armor, essentially shredding any infantry that happen to be nearby the tank - and leaving the tank vulnerable to shoulder-fired missiles, since such armor is by nature one-use.
More recent versions of reactive armor have become more complex, attempting to defeat kinetic penetrators from APFSDS type rounds as well as disrupting shaped-charge attacks. Although it is much more difficult (to a vanishingly low probability of success) to actually destroy the incoming projectile (and remember, you still have all that energy coming in unless you're firing the equivalent of a tank shell of energy out and manage to precisely cancel it) it is possible to 'wobble' it or disrupt its shape. Either of these results will increase the surface area of actual armor that the incoming round strikes, effectively lowering its penetrating power.
See also spaced armor, Chobham armor. | 
