The dissertation assesses the causes of peace and armed conflict between major powers and nearby states with inferior military capabilities. Although such conflicts have decisively shaped modern history, existing research has not explained why such unequal neighbor’s often self-select into war. This is puzzling since doing so threatens the weak neighbor’s survival and diverts the powerful neighbor’s attention and resources away from seemingly more important issues. I show that such conflict recurs in situations where leaders on both sides find assertive and militarized policies to be the best means of securing their own political survival. As the sociopolitical and strategic affairs of neighboring states are strongly interlinked, specific shifts in either side s domestic and external power politics can inadvertently threaten assets relevant to leadership survival on the other side. I identify three such shifts as the root causes of military conflict between unequal neighbors: 1) rising antagonistic demands by either neighbor’s power base; 2) strong neighbor security crises, provided the weak neighbor’s elite perceives the strong neighbor as hostile; 3) increasing weak neighbor alignment with major external states, provided the big neighbor’s elite perceives either of these states as hostile. I demonstrate the theory s explanatory power relative to liberal and realist models using a nested multi method research design. This includes a large-n, high-reliability test using logistic regressions on conflict onsets for all unequal neighbor dyad-years between 1816 and 1989. I also perform in-depth, high-validity tests using process-tracing on conflict variation between Austria-Hungary and Serbia (1878-1914) and between Ukraine and Russia (1992-2014). Data stem from primary sources of leadership deliberation and original policy documents and interviews conducted in Russia and Ukraine. The dissertation contributes to our understanding of security dilemmas in alliance politics, determinants of extended deterrence, political survival and elite perceptions in international security, and the determinants of Russo-Western security relations.