After years of steady growth and a pandemic-related explosion, online learning has become a common format for college courses. This shift has helped institutions by keeping the virtual door open during emergencies, broadening their pool of potential students, and decreasing brick-and-mortar operating costs. We know less, however, about how online learning affects student learning. While prior research has found negative or mixed effects, these are based on self-selected groups of students. College students typically choose their institution and course of study, including whether they take some or all of their classes online. This study is the exception, studying the United States Military Academy West Point, a unique institution established to educate future Army officers, where students have little control over their class schedules. More than 80 percent of the academic program is standard across all majors, and coursework is identical across instructors, who follow a unified curriculum with the same materials and graded events. These unique attributes allowed the authors to conduct a randomized controlled experiment during the fall 2020 semester, when West Point brought students back to campus but also used online instruction to limit class sizes, maintain social distancing, and slow the spread of COVID-19. The study compares coursework, grades, and results of a post-course survey among 551 students who were randomly assigned to online and in-person sections of Principles of Economics, a required class that most students take their sophomore year. The study finds that student learning, focus, and engagement suffer when instruction moves online. When the ways that West Point differs from traditional colleges and universities are considered, the findings are concerning. If online learning has negative effects at West Point, where students are among the nation's most disciplined young people, what does that mean for the typical student?