Force generation by actin assembly shapes cellular membranes. The mechanisms that govern the organization of cytoskeletal complexes to produce directional force in cells are not understood, particularly in the localized membrane deformations required for membrane trafficking. An experimentally constrained multiscale model shows that a minimal branched actin network is sufficient to internalize endocytic pits against membrane tension. Around 200 activated Arp2/3 complexes are required for robust internalization.