Planning tactical operations on topographic maps, for rescue or military missions, is a complex process conducted by interdisciplinary experts and involves the time-consuming derivation of 3D information from 2D maps, mostly solely executed by experienced professionals. Previous research repeatedly showed that virtual reality (VR) can convey spatial relationships and complex 3D structures intuitively. In this work, we leverage the benefits of immersive head-mounted displays (HMDs) and present the design, implementation, and evaluation of a collaborative VR application for tactical resource planning on spatial data. We derived system and design requirements from consultations with domain experts and observations of a military on-site staff exercise, a simulation-based training aiming to strengthen rapid decision-making and teamwork during a time of crisis. To evaluate our prototype, we conducted semi-structured interviews with domain experts who organized and observed field tests at different military staff exercises. The interviews support the proposed design of the prototype and show general design implications for planning tools in VR. Our results show that the potential of VR-based tactical resource planning is dependent on the technical features as well as on non-technical environmental aspects, such as user attitude, prior experience, and interoperability.