Why do similar armies deployed in the same peace mission with a similar level of material resources behave differently? Why are they effective in different ways? According to conventional wisdom, an army’s material resources, such as the number of troops it can deploy, its equipment, and budget more generally, determine their level of success and failure in peace operations. Yet, my analysis proves that the link between resources and success of peace and stability operations is, on average, not very strong. I argue, first, that this is because the dichotomy of success and failure of a mission does not always reflect a military organization’s 'peace operation effectiveness,' a new concept developed to evaluate peacekeepers’ practices. Second, I show that despite an increasing convergence among armies worldwide, and between Western ones in particular, soldiers still behave differently in the field. For instance, in two very different operations, the NATO mission in Afghanistan and the UN mission in Lebanon, the French and the Italian battalions deploy a similar number of soldiers and similar equipment and vehicles. In addition, they operate under the same NATO and UN procedures and identical rules of engagement, implementing similar doctrines, receiving similar training and deploying in areas with comparable threat levels. However, they behave very differently. In a second step I argue that it is the difference in the force employment on the tactical level that explains variation in effectiveness. But why do armies behave differently in peace operations? The third part of my argument contends that, in opposition to the prevailing sociological, military, and political factors, the most important source of variation in force employment are differences in the 'military culture' of different military units. In order to understand and test this argument, I conducted extensive fieldwork in Lebanon and Afghanistan between 2007 and 2008. This work lies at the crossroads between security studies and military sociology and makes an empirical contribution to debates about the role of ideational factors in the social sciences.