Do walls work? The effectiveness of border barriers in containing the cross-border spread of violent militancy

Since the end of the Cold War, walls, fences, and fortifications have been constructed on interstate borders at a rapid rate. It remains unclear, however, whether these fortifications provide effective security. We explore whether border fortifications provide security against the international spread of violent militancy. Although barriers can reduce the likelihood that militant activity diffuses across international borders, their effectiveness is conditional upon the roughness of the terrain on which they are built and the level of infrastructure development in their proximity. Barriers require intensive manpower to monitor and patrol, and so conditions like rough terrain and poor infrastructure render security activity more difficult. However, rebels and other militants prefer to operate in such difficult areas, ultimately reducing the effectiveness of barriers in containing the international spread of violent militancy. Analyses on newly collated data on interstate border fortifications within a global sample of contiguous-state directed-dyad-years show that border fortifications are only effective in limiting the diffusion of militancy in contexts in which states can plausibly monitor and police their borders. This paper has significant implications for the academic literatures on national security and intrastate conflict, and it also speaks to the broader policy debate over border walls and fences.