Students across disciplines struggle with sensemaking when they are faced with the need to understand and analyze massive amounts of information. This is particularly salient in the disciplines of both history and data science. Our approach to helping students build expertise with complex information leverages activity theory to think about the design of a classroom activity system integrated with the design of a collaborative open-source network-analysis software tool called Net.Create. Through analysis of network log data as well as video data of students' collaborative interactions with Net.Create, we explore how our activity system helped students reconcile common contradictions that create barriers to dealing with complex datasets in large lecture classrooms. Findings show that as students draw on details in a historical text to collaboratively construct a larger network, they begin to move more readily between small detail and aggregate overview. Students at both high and low initial skill levels were able to increase the complexity of their historical analyses through their engagement with the Net.Create tool and activities. Net.Create transforms the limitation of large class sizes in history classrooms into a resource for students' collaborative knowledge building, and through collaborative data entry it supports the historiographic practices of citation and revision and helps students embed local historical actors into a larger historical context.