Substitutes or complements?: The effects of alliances on military spending in major power rivalries

In the short‐term, the mechanisms to address security threats are largely confined to building up one's military and securing an alliance with another state. This paper seeks to assess the extent to which alliances substitute for or complement military buildups in major power rivalries over the period 1816–1976 and the conditions under which such effects occur. Overall, the results when considering behavior on a year to year basis indicated, at best, a modest complementary effect of alliances on arms spending increases. Nevertheless, such an effect was not apparent in all analyses and could not be attributed, mitigated, or exacerbated according to the type of alliance (e.g., defense pact versus entente) or the identity of the alliance partner (i.e., major versus minor power). When considering behavior across the rivalry as a whole, the results were clearer. Non‐aggression pacts substituted for arms increases in the context of high and increasing defense burdens. Defense pacts had a complementary effect when they were multiparty alliances that expanded the commitments of the major power rivals; this largely resulted from states entering alliances with small powers after 1945, with the resulting free‐riding by those small states.