The present article conducts comparative study of the foundations and sources of legitimacy for the rules governing the conduct of parties to Armed conflict in the two systems of international humanitarian law and Islamic jurisprudence. The main problem is the gap between the formal legality of these rules and their substantive legitimacy in Islamic societies, which has led to a crisis of effectiveness. Employing a comparative-analytical method, the research compares the two systems based on five indicators: "source and origin," "interpretability," "flexibility," "basis of obligation," and "enforcement guarantees." The findings reveal that in international law, the legitimacy of rules is grounded in legal positivism, state consent, and treaties, whereas in Islamic jurisprudence, legitimacy derives from the organic connection between the Qur'an, Sunnah, independent reason ('aql), and the objectives of Sharia (Maqasid al-Shariah), regarding the rules of war as divine obligations. Despite these fundamental differences in epistemic foundations, the two systems exhibit functional convergence in their ultimate goal of limiting violence and protecting civilians. The article concludes that the way out of the legitimacy deadlock lies in moving from a linear correspondence of rules toward teleological convergence through reliance on the doctrine of Maqasid al-Shariah and the strategy of norm localization. This approach introduces humanitarian rules in a jurisprudential framework to Islamic armed groups, paving the way for their broader acceptance.