On the global stage, Ukrainian President Volodymyr Zelensky has been consistently on message: In visits to Washington and other Western capitals, he has focused on keeping Kyiv supplied with more advanced arms, ammunition and financing. At home, however, he faces a human resources problem. The war is approaching the end of its second year, and Ukraine’s military needs more manpower to sustain a bloody war of attrition against Russia, a country with more than three times the population of Ukraine. In a recent essay, Ukraine’s top military commander, Valery Zaluzhny acknowledged that training and recruiting troops was becoming a serious...