“THE NUMBER of women married as children is staggering,” says Rachel Vogelstein of the Council on Foreign Relations. Around the world, nearly 5 million girls under the age of 15 get married each year (about 13,000 a day) and some of those are as young as 8 or 9. This is, obviously, a gross human-rights violation: girls are forced to leave their families, they are subject to sexual and other physical abuse, and they bear children while still in childhood themselves. As Vogelstein points out in her new study, “Ending Child Marriage,” in some cultures where girls undergo genital mutilation as they approach puberty, they are also pressured to marry almost immediately. Beyond the consequences for the young women, there is a corrosive effect on their communities, and Vogelstein argues that fact should be a serious concern for American foreign policy. The practice traps girls—and traps their children—“in a cycle of poor health, illiteracy, poverty, and violence,” writes Vogelstein. In its own interest the United States needs to understand that addressing child marriage is “a cost-effective and strategic imperative,” she writes, and Washington should make it a priority.