While the Europeans were discussing integration, other nations experienced long conflicts over borders. In some of the latter contexts, the significance of borders was underscored by escalated border policing through the erection of barriers. Although barrier construction is not a new phenomenon, an increasing number of nation-states are launching barrier projects along their borders. While in all cases the concerned nation-states claimed these barriers were provided security, scant attention was given to the actual security outcomes of these constructions. This research provides answers to the questions: what accounts for the different security outcomes of border barriers? How can barriers differ? And why? How does variation in barriers affect the nature and number of non-state actors' attacks? When do violent groups have tactical shifts and tactical innovations in the context of a barrier? This work moves beyond the conventional perspective on barriers that see them as successful defensive security measures. Instead, it develops the Fortification-Cooperation model that explains why the level of cross-border militant attacks change, when violent groups shift their tactics and when they innovate. I argue that security cooperation on both sides the border limits violent activities locally, which in turn restricts their access to militant resources. Presence of these resources is central for launching more attacks and for introducing tactical innovations. In turn, barriers impose restrictions to movement and increase the cost of certain attack. In this context, motivated violent groups substitute their commonly employed attack tactics for other types of attacks that can be sustained despite the presence of the barrier. Using a newly constructed qualitative and quantitative datasets on Palestinian attacks against Israel and barrier construction between 1990 and 2010, this study finds that the empirical record does not provide support the existing common explanations about the outcomes of barriers and that the interaction of cross-border cooperation and fortification is a key determinant of the number and nature of cross-border militant attacks. This work has significant implications for many states that built, are building, or are considering the construction of barriers on their borders since according to this research, a barrier without cross-border security cooperation would not be efficient at diminishing or decreasing cross-border violence. Additionally, violent groups' access to military resources is an important factor that should be taken into consideration when a barrier is built. Again, cross-border cooperation plays an essential role in restricting these resources, which would lead to less violence. In fact, in some cases, cooperation alone may result in similar outcomes to the combination of fortification and cooperation, which raises questions about the utility of barriers to begin with. / text