This paper analyses how democratic legislatures oversee the military, using Canada as a case. The paper argues that the tendency to engage in intrusive oversight versus reactive oversight is shaped by institutional structures and party preferences. Canadian institutional structures discourage parliamentary defence committees from engaging in intrusive oversight of the armed forces to achieve policy influence, and encourage opposition parties to focus on reactive oversight efforts that complement their vote-seeking preferences. Vote-seeking, the paper argues, incentivises opposition parties to be public critics of the government's handling of military affairs, rather than informed but secretive monitors of the armed forces. The paper then addresses a key case where the opposition was able to use an exceptional constitutional power of the House of Commons to force the executive to disclose classified information regarding the military: detainee transfers by the Canadian Armed Forces in Afghanistan. This case highlights the trade-offs that parliamentarians face when they demand information to perform more intrusive oversight of the armed forces. This suggests that party preferences are a significant, yet understudied, aspect of how legislatures vary in their oversight of the military.