Why did different regimes react differently to the mass uprisings that shook the Middle East and North Africa in 2010 and 2011? Why did the personalist presidencies of Husni Mubarak in Egypt and Zine al-Abidin Ben Ali in Tunisia collapse only weeks into the uprisings while Syria’s Bashar al-Assad still holds onto power and Yemen’s Ali Abdallah Salih could negotiate his way out of office? Focusing on the cases of Egypt, Syria, Tunisia and Yemen, this thesis is an attempt to answer this question. The central argument of this thesis is that military elite behavior shaped regime trajectories in the Arab Spring. Where the armed forces as an institution defected from the incumbent, the presidency immediately collapsed; where at least some military elites remained loyal, the respective chief executives survived in office for a significantly longer period. I develop an explanation that focuses on the presence of regime cronies within the military leadership. Where such cronies exist, the costs of defection increase for all members of the officer corps. Since the loyalty of cronies appears as a forgone conclusion, defection would likely lead to confrontation within the military. In other words, the absence of crony officers is a necessary condition for the cohesive defection of the armed forces from authoritarian presidents. Empirically, the fact that there were no crony officers in their respective militaries enabled the Egyptian and Tunisian armed forces to defect from their commanders in chief without endangering their internal cohesion. In Syria and Yemen, on the other hand, the defection of the armed forces as an institution was not an option given the fact that key units in both militaries were controlled by officers closely connected to the president. The result was the swift collapse of personalist presidencies in Egypt and Tunisia and the escalation of conflict in Syria and Yemen. This thesis traces the emergence of patterns of political-military relations in Egypt, Syria, Tunisia and Yemen from regime foundation in the 1950s and 1960s to the uprisings of 2010 and 2011. I argue that path dependent processes of institutional development link patterns of political-military relations at the outbreak of the uprisings to the dynamics of regime foundation in the early 20th century. While the institutional form of the founding regimes that II emerged in the 1950s and 1960s was a function of the composition of regime coalitions, the patterns of political-military relations that shaped regime trajectories in 2011 were shaped by attempts to reproduce these initial institutional features over time and under changing environmental conditions. The initial role of the armed forces in founding regimes was determined by whether or not the regime coalition had drawn institutional support from the military. Where this was the case as in Egypt and Syria, the military developed into a central regime institution, whereas the armed forces remained marginal in Tunisia and institutionally weak in Yemen. These initial differences were reproduced in the context of a period of institutional and economic reform from the second half of the 1970s onwards. While all four regimes succeeded in reining in the military, they used different strategies that had different and partially unintended consequences. In Egypt the depoliticization of the military was sugarcoated by the emergence of a parallel ‘officers’ republic’ that ensured substantial military autonomy, in Syria the armed forces were controlled via a system of praetorian units, while in Tunisia the military remained marginal but largely independent from the regime and in Yemen tribal dynamics prevented the army from developing into a strong institution. These processes all fulfilled their primary goal of ensuring that the armed forces would not actively intervene in politics. At the same time, however, they produced different incentive structures for military elites confronted with regime threatening protests.