A countermeasure is a system (usually for a military application) designed to prevent weapons from acquiring and/or destroying a target. Countermeasures may function by concealing the sensory signature of the target, or deceiving or disrupting the target detection systems of the attacker.

Countermeasures can act against target acquisition systems that depend on electronic, thermal, infrared, optical, or radar technology. Countermeasures are most popularly associated with aircraft defense, with examples including metallic foil chaff to disrupt radar detection, decoy flares to disrupt infrared, and electronic countermeasures to disrupt other targeting and communications systems. However, land and sea-based forces can also use such countermeasures as well as smoke-screens that can disrupt laser ranging, infrared detection, laser weapons, and visual observation.

Countermeasures are a complicating factor in the development anti-ballistic missile defense systems targeting ICBM's. Like aircraft, ICBM's theoretically could evade such systems by deploying decoys and chaff in the midcourse phase of flight. Novel proposed chaff mechanisms describe the creation of a "threat cloud" by deploying of large alluminized PET film balloon which could conceal a warhead among a large number of inert objects having similar radar profiles.

In the wake of missile attacks against civilian passenger and cargo airliners in the early 2000's, various agencies investigated the feasibility of equipping countermeasures chaff and flares. Many commercial carriers found the estimated price of countermeasures to be too costly. However, the Israeli airline El-Al, having been the target of a failed missile attack in Mombassa, Kenya in 2002, began equipping its fleet with radar-based, automated flare release countermeasures from June 2004Missile defense for El Al fleet, CNN, May 24, 2004. Accessed July 18, 2006.. This caused concerns in some European countries, which proceeded to bar such aircrafts from landing at their airportsEurope objects to El Al's anti-missile shield, Ynetnews, Feb 26, 2006. Accessed July 18, 2006..

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