This interdisciplinary thesis analyzes the role and use of international law in international security. Based on four papers, the thesis offers theoretical and analytical contributions regarding states’ conceptualization of war and peace under international law; the design of global disarmament treaties; states’ political use of the Treaty on the Prohibition of Nuclear Weapons; and the legal effects of the multilateral negotiations on the prevention of an arms race in outer space. The thesis uses legal analysis and the prism of international law’s function to constrain, screen, and authorize state behavior for making broader theoretical, normative, and empirical claims. The thesis finds that states continue to use international law in international security because it serves their objectives despite - or even because of - its limitations.