This study aims to identify the reasons that justify the state’s use of military force in self-defense according to Islamic Sharia, in comparison with the justifications in contemporary international law. In this study, we have adopted the inductive method by tracing the particulars related to the topic in their sources, the deductive method in extracting rulings from Sharia and legal texts, and the comparative method in comparing the rulings of Sharia with the law. The study concluded that the justifications for war of various types in Islamic Sharia are limited to two cases: legitimate defense and the achievement of justice. This includes self-defense, repelling injustice, supporting the oppressed who are unable to defend themselves, ensuring freedom of belief, ensuring the safe delivery and dissemination of the Islamic message, cooperating with non-Muslims in fighting a common enemy, and repelling aggression against others. In contrast, international public law limits justifications to cases of legitimate defense and Security Council resolutions, and the use of military force is not justified except in the case of an armed attack. The study also concluded that offensive warfare is considered nothing more than preventive defense.