Do alliances allow states to share defense burdens and reduce military spending? Despite expectations that alliances should lead to decreased military spending, the empirical record offers mixed findings. We argue that not all alliances are reliable; thus, only allies that receive signals of reassurance will rely on the external security of allies and subsequently reduce their military spending. Compared to states that do not receive additional signals, these reassured allies will have greater confidence that an ally will come to their aid. As a result, third-party aggressors are deterred and the demand for military spending will decrease. We test this argument with an analysis of US signals of support, alliance commitments, and military spending. We find that American alliances without additional signals of support have a negligible effect on military spending. Yet, we observe that alliances are negatively associated with military spending when signals of support are present. Additional tests indicate that alliance commitments, coupled with strong US signals, are also associated with lower military spending in the rivals of US allies. Our results potentially help explain the mixed evidence in the arms-versus-allies and burden-sharing literatures and further demonstrate that extra-alliance signals play an important role in the practice of International Relations.