Given that Marianne Moore never married and apparently never fell in love, readers have long puzzled over the inspiration for "Marriage." Why would the least autobiographical member of her famously impersonal generation devote her longest poem to such a subject? By tracing the friendship between Moore and Scofield Thayer through archival sources, this essay reveals for the first time that Thayer proposed to Moore in 1921 and that she wrote "Marriage" as an angry rebuke to him. Reading the poem in this context shows it to be not simply "a little anthology of statements that took my fancy," as Moore later described it, but rather an instance of "by-play" being "more terrible in its effectiveness than the fiercest frontal attack." The poem presents an impassioned indictment of all loveless marriages but allows for those rare marriages that exemplify the paradox of "Liberty and union, now and forever."