Using lasers, they guide lightning to their targets:

“For very powerful and high intensity laser pulses, the air can act like a lens, keeping the light in a small-diameter filament,” said Fischer. “We use an ultra-short-pulse laser of modest energy to make a laser beam so intense that it focuses on itself in air and stays focused in a filament.”

To put the energy output in perspective, a big filament light bulb uses 100 watts. The optical amplifier output is 50 billion watts of optical power, Fischer said.

“If a laser beam is intense enough, its electro-magnetic field is strong enough to rip electrons off of air molecules, creating plasma,” said Fischer. “This plasma is located along the path of the laser beam, so we can direct it wherever we want by moving a mirror.”

“Air is composed of neutral molecules and is an insulator,” Fischer said. When lightning from a thunderstorm leaps from cloud to ground, it behaves just as any other sources of electrical energy and follows the path of least resistance.

“The plasma channel conducts electricity way better than un-ionized air, so if we set up the laser so that the filament comes near a high voltage source, the electrical energy will travel down the filament,” Fischer elaborated.

A target, an enemy vehicle or even some types of unexploded ordnance, would be a better conductor than the ground it sits on. Since the voltage drop across the target would be the same as the voltage drop across the same distance of ground, current flows through the target. In the case of unexploded ordnance, it would detonate, explained Fischer.

Picatinny engineers set phasers to 'fry' | Article | The United States Army.