This Article explicates the under-conceptualized jus ad bellum proportionality principle in international law, and then assesses Israel’s compliance with proportionality during its use of force against Hamas in Gaza following the October 7, 2023, attacks. Using our proposed three-step analytical approach that we argue should guide jus ad bellum proportionality assessments, this Article finds, first, that Israel had a right of self-defense, and that Israel’s strategic objective of removing Hamas from political power and eliminating its military capabilities fell within the legitimate boundaries of self-defense. Second, however, this Article finds that Israel’s strategic objective required means that would foreseeably cause excessive damage and harm in this context, due to circumstances in Gaza that were wellknown to Israeli officials. Israel accordingly violated proportionality in pursuing this strategic objective. Third, satellite radar imagery analyses demonstrate that Israeli operations in fact caused extensive damage during the first year of the war that exceeded what was required for Israel’s self-defense, in violation of the proportionality principle. This Article concludes by discussing possible pathways for recourse under international law, and highlights the need for future rigorous analyses of proportionality in ongoing and prospective uses of force that can guide State behavior and promote compliance with international law.