Geographic data are used by United Kingdom (UK) defence for purposes including peacekeeping, humanitarian aid and disaster relief, and fighting wars. The geographic extent of defence data covers the world, with greater focus directed towards areas considered to be of current interest. Traditionally, these data have been officially sourced, e.g. via National Mapping Agencies, but there is now increasing interest in the potential of crowdsourced geographic data to supplement authoritative data where they are not available, outdated or incomplete. Volunteered geographic information (VGI) and social media have the potential to provide this needed missing information. This paper presents initial work carried out in identifying the potential of crowdsourced geographic information in defence. We first provide a short description of the role of UK defence and review the existing literature on crowdsourced geographic information in defence, as well as generic VGI quality assessment methods. We then explore the potential of crowdsourced data in real-world applications: the conflation of VGI and social media with official data for effective decision-making in war zones, and the potential for crowdsourcing to increase effective collaboration between machines and humans in disaster situations. Based on our review, we outline specific research challenges for deploying crowdsourced geographic information in defence, focussing on data quality and fitnessfor-purpose assessment. Defence-specific constraints include the need for rapid quality assessment processes and the need to communicate high-quality information effectively in situations where rapid decision-making is required. Ethical issues are also of fundamental importance.