This is a tipping point, because what you can do with 150 or 300 kW also depends on what you’re trying to protect. If the goal is to knock down a supersonic anti-ship cruise missile (the focus of some long-running Navy programs), there are two problems. Water in the atmosphere—whether humidity, rain or spray—attenuates laser energy, and a damaged cruise missile can still hit the target in the form of a ton of high-speed burning trash. But if the target being protected is an evasively maneuvering aircraft, it will often be in clear, dry air; and it is enough to destroy the missile’s seeker, put a hole in the radome or weaken the motor tube to cause a miss, even at well-sub-kilometer range.

Shooting a small, fast target is a laser’s forte, because what makes it new as a weapon is its instantaneous impact. The century-old basic air warfare problem—figuring out where the target is going to be when the bullet or missile gets there—is over.

Two laser-armed fighters in a formation could act as escorts, with 20 quick shots to defeat a pop-up threat. A bomber on a maritime strike mission would be able to penetrate the outer zone of a hostile fleet’s defenses. Countermeasures are expensive: mostly they mean trying to harden the missile, invariably adding weight.