Do alliances allow states to share defense burdens and reduce their 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 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 U.S. signals of support, alliance commitments, and military spending. We find that alliances without additional signals of support have a negligible effect on military spending. Yet, we observe that signals of support are negatively associated with military spending among U.S. allies. Additional tests indicate that strong U.S. signals, coupled with alliance commitments, are also associated with lower military spending in the rivals of U.S. allies. Our results potentially help explain the mixed evidence in the arms-versus-allies and burden-sharing literatures and further demonstrate that signals play an important role in the practice of international relations.