Atlas leaves for the Marines is the pivotal separation in Lily Bloom's adolescence, the moment her first love enlists in the military four years earlier than planned, promising to return only when his life is good enough for her. Rooted in Atlas's homelessness and the town's condemnation of him, this departure becomes the defining absence that shapes Lily's understanding of love, class, and the cycles she later must break. It is the event that transforms Atlas from a present, protective presence into a remembered promise, and it sets the terms for every reunion that follows.