Recent work has begun exploring the effects of foreign military deployments on hoststate foreign policies. However, research mostly focuses on dyadic relationships between major powers and host-states, ignoring the broader regional security environment of host-states. We develop a theory of spatial hierarchies to understand how security relationships throughout the region surrounding the host-state affect hoststate foreign policy. Using data on US military deployments from 1950–2005, we show that regional security considerations condition how host-states respond to the deployment of military forces to their territory. Consequently, regional analyses are fundamental in understanding monadic and dyadic decisions about security, alliance behavior, and conflict.