War was just an experiment for two of the U.S. military’s oldest and most unusual warplanes. A pair of OV-10 Broncos—small, Vietnam War-vintage, propeller-driven attack planes—recently spent three months flying top cover for ground troops battling ISIS militants in the Middle East. The OV-10s’ deployment is one of the latest examples of a remarkable phenomenon. The United States—and, to a lesser extent, Russia—has seized the opportunity afforded it by the aerial free-for-all over Iraq and Syria and other war zones to conduct live combat trials with new and upgraded warplanes, testing out the aircraft in potentially deadly conditions before committing...