With America’s drone technology a disappointment on the battlefield, defense startups have joined forces with Ukrainian manufacturers to build better, war-proven aircraft for the U.S. military. U.S. startups have spent billions of venture-capital dollars in hopes of developing the small drones that the Pentagon says it needs for future conflicts, but many have produced only expensive aircraft that don’t fly very well. Ukrainian drone makers, meanwhile, have mastered mass-producing drones despite limited resources and are looking for new customers and capital. Now, the two sides are coming together, and the unlikely pairing is getting attention from the Defense Department. Southern...