This is the untold story of America’s hidden role in Ukrainian military operations against Russia’s invading armies. On a spring morning two months after Vladimir Putin’s invading armies marched into Ukraine, a convoy of unmarked cars slid up to a Kyiv street corner and collected two middle-aged men in civilian clothes. Leaving the city, the convoy — manned by British commandos, out of uniform but heavily armed — traveled 400 miles west to the Polish border. The crossing was seamless, on diplomatic passports. Farther on, they came to the Rzeszów-Jasionka Airport, where an idling C-130 cargo plane waited. The passengers...