The recent development of the Indo-Pacific region is characterised by the changing balance of power and the emergences of new forms of security cooperation. The Quadrilateral Security Dialogue (QSD) between Japan, the United States (U.S.), Australia and India came back into existence in 2017 after their failed attempt in 2007-2008. This thesis attempts to investigate what factors explain the re-emergence of the QSD by synthesising several alignment/alliance theories in International Relations (IR). Given the previous research on the QSD and theoretical discussions, this thesis points out the two key factors that contributed to the re-emergence of the QSD: the shared threat perception towards China and the shared objectives in the Indo-Pacific region. The content analysis of a number of official policy documents and press statements revealed that Japan, the U.S., Australia and India have increasingly perceived China as a threat and coordinated their policy objectives in the Indo-Pacific region under the concept of the Free and Open-Indo Pacific.