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.

联盟是否允许各国分担国防负担并减少军费开支？尽管预期联盟会导致军费开支减少，但经验记录提供的结果喜忧参半。我们认为并非所有的联盟都是可靠的；因此，只有收到保证信号的盟国才会依赖盟国的外部安全，从而减少其军费开支。与没有收到额外信号的国家相比，这些放心的盟友将更有信心相信盟友会来帮助他们。其结果是，第三方侵略者受到威慑，对军费开支的需求将减少。我们通过分析美国的支持信号、联盟承诺和军费开支来检验这一论点。我们发现，没有额外支持信号的美国联盟对军费开支的影响可以忽略不计。然而，我们观察到，当出现支持信号时，联盟与军费开支呈负相关。其他测试表明，结盟承诺加上美国发出的强烈信号，也与美国盟友竞争对手的军费开支降低有关。我们的结果可能有助于解释武器对抗盟友和负担分担文献中的混合证据，并进一步证明超联盟信号在国际关系实践中发挥着重要作用。还与美国盟友的竞争对手较低的军费开支有关。我们的结果可能有助于解释武器对抗盟友和负担分担文献中的混合证据，并进一步证明超联盟信号在国际关系实践中发挥着重要作用。还与美国盟友的竞争对手较低的军费开支有关。我们的结果可能有助于解释武器对抗盟友和负担分担文献中的混合证据，并进一步证明超联盟信号在国际关系实践中发挥着重要作用。