Background: Despite its numerous benefits, dialogic discussion seldom occurs in secondary history classrooms. To examine ways to promote it, this study compared two ninth-grade classes' productive disciplinary engagement (PDE) in a whole-class discussion on the Gulf of Tonkin incident. Methods: The two class sections were comparable academically but had experienced slightly different implementations of the curriculum. Analytic codes tracked discursive moves within each discussion (e.g., authentic questioning), as well as shifts in discursive style (e.g., from monologic to dialogic). Findings: Analysis suggests that both classes employed sound historical reasoning throughout, but that the third-period class grew more productively engaged. The disparity traces back to the days spent preparing students for discussion, which in third period served to construct a more compelling problem to explore, and to position students as more knowledgeable authorities. Yet even in that class, student dialogue emerged late in the discussion, after segments devoted to establishing textual evidence and answering student questions. Contribution: Based on these findings, a model is presented for promoting PDE in historical discussions that emphasizes the distribution of intellectual authority and the provision of sufficient time and resources.