The U.S. military has taken ownership of a prototype 300-kilowatt laser -- the first of three different variants of a high-energy weapon with cruise-missile killing power -- marking a key milestone in the Defense Department program to scale up directed-energy technology. On Aug. 8, Lockheed Martin delivered to the office of the under secretary of defense for research and engineering the most powerful laser the company has ever built: an electric 300 kW-class laser designed for tactical operations. “Over the...