In this paper we use cross-national survey data in combination with a novel, large-scale survey experiment fielded in three countries to understand and disentangle how foreign military deployments impact defense policies of host states. The theory of free-riding in military alliances postulates that member states piggyback on security commitments by guardian states. Because foreign military deployments (in the form of troops or nuclear weapons) render such promises particularly credible, guardians such as the U.S. regularly threaten withdrawal from NATO member states in order to undermine alleged free-riding. In this paper, we examine the micro-foundations—and mechanisms—of this logic. First, we utilize cross-national survey data from the European and World Value Survey to demonstrate that foreign military deployments do indeed decrease the significance citizen attribute to national defense. However, we argue that free-riding critically implies that foreign military deployments positively affect subjective feelings of security and reduce threat perceptions (due to assurance and extended deterrence). By contrast, if citizens suspect the guardian’s motives to be primarily self-serving by advancing foreign policy that may not be in the interest of host states, heighten the risk of host ‘entrapment’ in major (nuclear) war—in which national defense would be futile. Thus, in a second step we disentangle and trace these mechanisms by devising a survey experiment (n=6000; conducted in Germany, Czechia and the Netherlands) that (1) measures perceptions on the guardian's motives, (2) experimentally manipulates foreign military deployments, (3) examines threat perceptions, and (4) captures attitudes towards national defense.