This chapter presents a typology of third-party security commitments. These commitments are often formalized in the terms of interstate military alliances. The security commitment observed in most alliance agreements consists of a promise by one country to provide military support to another country subject to some condition. It is a statement taking the form “if x then y,” where y is the promise of some kind of military support and x is the condition activating the military response. In principle, both x and y can vary a great deal. A country may promise complete support to its ally, or it may specify a given level of military support or even only the possibility of military support. It can also promise to remain neutral. Leaders can also specify numerous possible conditions or combinations of conditions to trigger their promise to deliver y. These conditions comprise the content of x and can range from highly specific (e.g., an adversary's attack and a protégé's nonintervention) to nonspecific (e.g., any conflict). Additionally, the conditions might stipulate the objective of the conflict, such as those contests limited to the water's edge (e.g., only if an adversary tries to gain concessions from the protégé) or to more aggressive pursuits (e.g., an adversary's refusal to concede some territory).

To build a theory of commitment design, it is essential to know what kinds of commitments countries form in practice and the circumstances leading to each type of commitment. In particular, we might ask whether there is any systematic variation in x and y in the empirical record. Scholars have long studied interstate military alliances and have created models, categories of types of military alliances, and datasets based on these categories to gain traction on the issue. A typology is a useful way to begin theorizing about alliance commitments, because it helps us identify and analyze the different mechanisms decision makers design to address threats. However, existing theories and typologies are limited because they do not fully capture the commitments in the historical record.