Bilateral intelligence cooperation has received increased attention in recent years, thanks in part to its publicly acknowledged role in supporting sovereign states in their efforts to counter transnational threats. Until recently most of the scholarship on this topic has been descriptive and atheoretical, with a tendency to treat known examples of cooperation as unique occurrences, rather than as instances of a broader phenomenon. The aversion to theory surrounding bilateral intelligence cooperation has inhibited efforts to develop generalized and contingent explanations about it, such as why it occurs in the first place and under what conditions it is most likely to flourish and atrophy. This dissertation seeks to address these gaps in the Intelligence Studies literature by leveraging insights from two theoretical traditions from International Relations--the dyadic democratic peace and relational contracting perspectives--to develop candidate explanations of why certain pairs of states engage in greater degrees of intelligence cooperation over time than others. Two historical case studies--the Anglo-Soviet and Anglo-American military intelligence relationships during World War II--are presented and analyzed with a view to assessing the relative strengths and weaknesses of each explanation. Through the use of qualitative research methods (i.e., the congruence method and testing of observable implications) and the development of new numeric measurements to capture the depth of intelligence cooperation over time, the author finds that the normative dyadic democratic peace hypothesis and its posited causal mechanism of the regime recognition dynamic are largely corroborated in both cases, thus suggesting that they warrant further consideration as an explanation of the depth of bilateral intelligence cooperation. By contrast, the relational contracting hypothesis and its posited causal mechanism of willful hierarchy are not well supported in either case, thus raising questions about their applicability and generalizability to the larger universe of cases.