That’s only for naturalized citizens though, not people who have dual citizenship through birth. For example, my husband is Korean. If he were to choose to become a US citizen through naturalization, he would have to renounce his Korean citizenship. My children, however, could maintain both US AND Korean citizenship. Since they are boys and Korea still has mandatory military service for men, they would have to renounce their Korean citizenship if they did not want to do military service, but they could continue to maintain dual citizenship if they chose to fulfill that requirement.