In 2015, leading up to the Paris Climate Conference, India faced intense scrutiny over its role in either securing or scuttling a global climate deal. On the first day of the climate talks India and France jointly announced the International Solar Alliance (ISA), and two weeks of hectic negotiations culminated in the adoption of the Paris Agreement. In less than two years, even as multilateral climate negotiations were weakening with the United States announcing its withdrawal from the Paris Agreement, the ISA - led by India and backed primarily by developing countries-became a legal entity. Vyoma Jha presents a case study of the creation of the ISA as a treaty-based international organization. Drawing on the political economy approach in the study of international law, this book identifies the politics, players, and process behind the making of the ISA. It uses mixed methods to analyse the politico-legal issues involved in the need for a new treaty-based international organization and finds that the changing political leadership in India marked a shift in domestic climate politics, particularly around solar energy. Against the backdrop of multilateral climate negotiations, the political leadership empowered India's new international rulemaking stance. Jha offers an in-depth account of the treaty-making process to argue that it marks an innovation in the structure of international organizations. The ISA is best described as 'soft law in a hard shell' because it uses the legal infrastructure of a treaty while relying on the social structure of participating actors for its future implementation. Empirical evidence suggests that three factors explain the treaty structure of the ISA: India's leadership role in the treaty-making process, the early involvement of non-state actors, and the preference of developing countries for legal form. Ultimately, the book illustrates a new kind of Indian economic diplomacy, making the ISA the first deliberate instrument of India's foreign policy on climate change and energy.