The Boeing YAL-1 Airborne Laser, is a laser weapon installed on board of a Boeing 747-400F.
It's a system considered essential to make a "Anti-Missile system" of the United States, because these aircraft have the capacity to destroy ballistic missiles in long ranges, yet in the launching of these missiles. Besides the defensive capacity, these airships operating a laser cannon, have a strategig offensive capacity, because they could realize high precizion attacks, with low level of side effects, in most of times, it could be negated by the attacker, because there won't would be indications of who could have committed the attack, different of what could occur with bombs or missiles.
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The original article can be found at Boeing YAL-1 and the edit history here.