Recently, growing intellectual energy has been spent on why states build walls and on walls’ effectiveness in thwarting violence. There is some evidence that fences mitigate flows of transnational terrorism between contiguous states (Avdan and Gelpi 2017) and, that they can stem cross-border militancy (Braithwaite and Linebarger 2020). Others rebut that walls do little to blunt terrorism (Carter and Ying 2020). The empirical debate remains unresolved because scholarship has not yet taken full account of the networked nature of terrorist groups. Terrorist organizations operate across bases in multiple proximate states, and thereby skirt state surveillance and monitoring by funneling their operatives and resources across borders, planning and executing cross-border attacks, and targeting porous borders.
We propose that fences degrade groups’ ability to move transnationally by hurting their ability to move easily across bases and choking off the flows of resources and operatives across contiguous territory. Using a network approach, we identify all states linked through shared borders, and using a diffusion model, we test the effect of border fences on the cross-border spread of groups. We focus on six groups, the Islamic State (ISIS), Abu Nidal, the Kurdistan Workers' Party (PKK), the Palestinian Liberation Front, Hezbollah, and Al-Qaida. On balance, we find that fences decrease the probability that a terrorist group spreads to a neighboring country; however, this effect is not a panacea, which suggests that policymakers and academics need to consider the goals, tactics, and capabilities of specific groups rather than presume that fences provide a ubiquitous cure-all.