During development, wound healing and cancer invasion, migrating cell clusters feature highly protrusive leader cells at their front. Leader cells are thought to pull and direct their cohort of followers, but whether their local action is enough to guide the entire cluster, or if a global mechanical organization is needed, remains controversial. Here we show that the effectiveness of the leader–follower organization is proportional to the asymmetry of traction and tension within cell clusters. By combining hydrogel micropatterning and optogenetic activation, we generate highly protrusive leaders at the edge of minimal cell clusters. We find that the induced leader can robustly drag one follower but not larger groups. By measuring traction forces and tension propagation in clusters of increasing size, we establish a quantitative relationship between group velocity and the asymmetry of the traction and tension profiles. Modelling motile clusters as active polar fluids, we explain this force–velocity relationship in terms of asymmetries in the active traction profile. Our results challenge the notion of autonomous leader cells, showing that collective cell migration requires global mechanical organization within the cluster.