HOHENFELS, Germany — Chief Warrant Officer 2 Chris Lehr and his small team of Wiesbaden-based soldiers spent about a year working on homemade, 3D-printed drones they weren’t sure would work.

HOHENFELS, Germany — Dozens of U.S. soldiers specializing in electronic warfare gathered at an Army training ground in Bavaria in recent days, looking to gain mastery of high-tech systems that ...