Commitments to fight alongside other states are constituent elements of international orders. The very existence of many states depends on these commitments. Belgium exists in part, for instance, because British security depended on keeping invasion-launching territory out of the hands of great powers. Taiwan, South Korea, and many other states in Europe and elsewhere have been protected by US commitments. A case can be made that the continued existence of most states has depended at some time or another on third party guarantees.

Making such commitments credible poses special challenges, however. It is easier for a state to convince another that it will fight if attacked than that it will fight in defense of a third state. When the costs of war were so high during the nuclearized Cold War, for instance, why should the Soviets have believed that the US would defend Western Europe, much less the isolated Berlin outpost? This difficulty of convincing adversaries has led states to send troops abroad whose role, in the event of combat, is largely to “die heroically, dramatically, and in a manner that guarantees that the action cannot stop there” (Schelling 1966, p. 47). Thus, the US stationed a few troops in Berlin during the Cold War, and maintains 28,500 soldiers in South Korea today. If these troops were attacked, surely the domestic reaction would push a US administration towards intervention.

Yet, in other cases, countries rely on private diplomatic commitments. When Germany committed to defend Austria–Hungary, and Russia Serbia, prior to the First World War, it was what diplomats said behind closed doors that counted with friendly and hostile governments. Today, the US maintains a partial commitment to defend Taiwan without basing any troops on the island. Upon what does the credibility such promises depend?

In negotiations involving three or more states, the signaling mechanisms analyzed in Chapters 3 and 4 are often available alongside others that have been described in bilateral contexts. If the interests of states in a coalition are closely aligned, the coalition can often be thought of as a single actor. When more than two states are involved, however, the possibilities for costless diplomatic signaling actually increase.