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 specific war aim of removing Hamas from political power and eliminating its military capabilities fell within the legitimate boundaries of self-defense. Second, however, Israel’s specific war aim required means that would foreseeably cause excessive damage and harm in this context, due to circumstances in Gaza that were well-known to Israeli officials. Israel accordingly violated proportionality in pursuing this specific war aim. Third, satellite radar imagery analyses demonstrate that Israeli operations have in fact caused extensive damage, excessive in relation to Israel’s self-defense aim and violating proportionality.