COVID-19 shifted schools and colleges to online instruction with little causal evidence of outcomes. In the fall of 2020, we randomized 551 West Point students in a required Introductory Economics course across twelve instructors to either an online or in-person class. Final grades for online students dropped by 0.215 standard deviations; a result apparent in both assignments and exams and largest for academically at-risk students. A post-course survey finds that online students struggled to concentrate in class and felt less connected to their instructors and peers. We find that the shift to online education had negative results for learning.