Historians use a range of genres in presenting their subjects, yet educators have increasingly privileged argumentation to help novices to reason with historical content. However, the influence genre and content knowledge are relatively unmeasured in this discipline. To learn more, the authors asked 101 eleventh-grade students to compose an argument or a summary on the Gulf of Tonkin and analyzed basic and disciplinary reading and writing measures. Results indicate that students with adequate content knowledge performed better on a disciplinary reading measure when composing arguments, and students with limited content knowledge demonstrated greater comprehension when composing summaries. Students with more knowledge wrote more; however, students who wrote summaries were not disadvantaged in terms of level of historical thinking or overall quality. Last, providing students with disabilities with a simple reading accommodation afforded them the ability to participate in the disciplinary literacy task at levels comparable to their peers.