Surrounded by prisoners dressed in black, a man identified as Yevgeny Prigozhin, the head of a shadowy mercenary group, makes this pitch: Come fight in Ukraine and be rewarded with your freedom — if you can make it back alive. "Nobody goes back behind bars," Prigozhin, founder of the Russian private military company Wagner, can be seen telling men in a video from a Russian prison yard, per a translation from The Wall Street Journal's Yaroslav Trofimov. "If you serve six months, you are free. If you arrive in Ukraine and decide it's not for you, we execute you." He...