On October 7, Hamas conducted a devastating attack on Israel, where its militants murdered civilians, took hostages, and committed other atrocities. Hamas, however, did not accomplish this attack alone. At a minimum, Iran provided funding, weapons, and training, enabling the group to achieve unprecedented scale and complexity. What is less clear is Iran’s involvement in the conduct of the attack and the accompanying atrocities. That uncertainty has limited the international community’s response. Limits in international law and consensus regarding state actor responsibility for proxy actions preclude imposing meaningful costs on Iran without expanding the conflict. Now may be the time to overcome these limits, develop consensus, and strengthen legal regimes to constrain Iran’s destabilizing support for its proxies.