Towards a Network Theory of Alliance Formation∗

We propose the first network-based theory of alliance formation. Our theory suggests that, in addition to key state and dyad attributes already established by the literature, the evolution of the alliance network from any given point in time is largely determined by its structure. Specifically, we argue that closed triangles in the alliance network – where i is allied with j is allied with k is allied with i – produce synergy effects in which node-level utility is greater than the sum of its dyadic parts. This idea can be generalized to n-node closure, and, when considered along with factors which make dyadic alliance formation more attractive such as military prowess and political compatibility, suggests that the network will evolve towards a state of several densely connected clusters of states with star-like groupings of states as an intermediary stage. To evaluate our theory, we use the SIENA model for longitudinal network analysis and find that the roles of our network effects are robustly supported by the data whereas the effects of non-network parameters vary substantially between periods of recent history. Our results indicate that network structure plays a more central role in the formation of alliance ties than has been previously understood in the literature. ∗Sincere thanks to James Fowler, Justin Gross, Mark Pickup, Randolph Siverson, and Tom Snijders for their helpful comments on previous drafts of this paper. We propose a network-based theory of alliance formation with the goal of explaining how the global alliance network evolves during and following major shifts in the distribution of power. We show that the structure of international political alignments, reflected in the system of alliances, operates in a manner which can best be described as punctuated equilibrium: periods of stasis followed by rapid and major shifts in the configuration of the system. We observe that the system of alliances undergoes major changes during and immediately following punctuations in world politics and propose a network-based theory to explain the means by which a new equilibrium state evolves. Our theory is network-based in so far as we draw not only on monadic and dyadic determinants of alliances, but focus our theoretical contribution on the complex interdependencies of the alliance network. This formulation allows the indirect effects of alliance ties to be felt throughout the network and makes specific predictions for how the structure of the network affects tie formation. We proceed by first arguing that the basic orientation of states to one another can be observed in the system of military alliances. We take military alliances – offensive and defensive pacts as well as consultation agreements – to indicate, at minimum, a signal of peaceful intentions between countries, a desire to coordinate foreign policies to some extent, and potentially the expectation of military support in the event it is needed. Second, we demonstrate that changes in the structure of alliances seem to operate by punctuated equilibrium. To do so, we identify periods of stasis and major transformation in the system of alliances by examining the year-to-year change in its structure; the periods we identify in this manner are in keeping with traditional interpretations of major changes in world politics. Third, we discuss the dyadic utility of alliances with particular attention to the role of military power and political compatibility between potential allies. Fourth, we explicate foundational element of our theory: the idea that triadic closure – a condition where three states are all allied to each other – produces a synergy effect among the member states such that the utility derived by each is more