Walls and Strategic Innovation in Violent Conflict

Governments build walls to curtail a range of illicit activities like immigration, crime, and terrorism. We argue that while physical barriers effectively prevent specific unwanted behavior, they induce actors to respond strategically and develop new tactics, changing the nature of illicit activity and leading to new threats. We test this argument in the context of Israel’s security barrier. Using an instrumental variable unrelated to the underlying threat of attack, we analyze short-term changes in the barrier’s porousness. Terror attacks in Israel are less likely when the barrier is more secure. However, we also observe evidence of changing strategies. Attacks are most likely immediately after the government eases temporary restrictions on movement, suggesting that previously-planned attacks were delayed, not prevented. Furthermore, when the barrier is more secure, terrorists select weapons that are less affected by it and carry out attacks in systematically different locations. Ultimately, walls’ impacts on any challenge depend not just on how well they prevent movement but also on illicit actors’ strategic responses.