One path to improving adolescents' literacy skills is to integrate reading and writing into the content areas in which such work occurs. Although argumentative writing has been found to help students understand historical content and transform information, scholars do not know the influence of specific task structures on students' writing or historical reasoning. To learn more, the authors administered four document-based writing tasks on the origins of the Cold War to 101 students from 10th or 11th grade. Using multiple regression, the authors found that writing tasks explained 31% of the variance in the quality of students' overall historical reasoning after accounting for differences in students' background. A closer analysis of different aspects of historical reasoning using a different rubric (and as analyzed using MANOVA) indicated that students' skill in recognizing and reconciling historical perspectives significantly improved with writing tasks that asked them to engage in sourcing, corroboration, and causal analysis. The task that asked students to imagine themselves as historical agents and write in the first person was significantly different and resulted in the lowest mean essay scores. (Contains 7 tables and 1 note.)