What did it mean for poor and middling men and women to take up arms against their government? How did they negotiate competing claims for their participation in civil war and what consequences confronted them? This article analyses the crown’s investigation of its opponents following the 1321–22 civil war, comparing its predecessor of the Montfortian civil war (1263–67), to examine how the king, justices and juries tackled these questions. It demonstrates how the crown rooted the summary conviction and execution of Thomas of Lancaster and other noble insurgents in common law procedure; then, at the King’s Bench and a special inquiry in the Welsh Marches, re-framed treasonous offences to tackle non-noble insurgents; then, fearing a new uprising, instrumentalised the common law’s machinery to gather military intelligence. The crown recognized the agency of subjects across society in civil war and juries were ideally placed to investigate it; they also weighed subjects’ culpability, balancing obligation to the king against the mitigating realities of coerced participation in war. Thus, juries and the communities who informed their verdict were invited to engage with the ethical and legal dilemmas of civil war. This article thus presents a people’s history of treason.