Proportionality and the Use of Force: International Law and Shifting Attitudes

• In Event: Uncertain Transatlantic Relations: Domestic Challenges in Interconnected Spaces

Abstract

This article contributes to the ongoing question of how international law—particularly customary international law (CIL)—affects decision-making regarding the use of force. A core debate in international law is whether binding law (like treaty commitments) is more powerful than non-binding law (like CIL). This article uses an experimental approach to examine respondents’ attitudes when introduced to the principle of “proportionality,” in which civilian harm must be proportional to the anticipated military advantage. This experiment was fielded first on an MTurk U.S. sample in 2017, which found that CIL can significantly decrease support for a potentially violative use of force, but that such information should be paired with some international monitoring authority. The survey was replicated on a Lucid sample in 2023 to test how American attitudes toward “international legal principles” have shifted across presidential administrations and in the face of increasing challenges to the liberal international order.

Author

• Melinda Haas, University of Pittsburgh