The present thesis analyses and clarifies the phenomenon of hybrid warfare. While the term has established its place in the political lexicon, current definitions and explanations are inadequate and unhelpful. This thesis addresses that shortfall by providing a concise strategic definition and a unifying theory of hybrid warfare. As a thesis grounded in the Realist tradition, the analysis focuses on the strategic implications with a view to aid in practical policy-making. Following a theoretical examination of the context and the intellectual history of the term, hybrid warfare is defined as a deliberately opaque blend of conventional and unconventional warfare. The rules of the international order and its 'guardian powers' are a key to understanding hybrid warfare and this thesis provides that understanding through the concept of the quinity. Based on the trinity, a concept emanating from Clausewitzian thought, the quinity blends traditional notions of war with the contemporary international order. The unique set of circumstances which such a combination creates is then combined with the proposed definition and operationalised through an examination of the defence policies and doctrines of the leading global powers. Hybrid warfare, whether practiced by state-like actors like Hezbollah, or states like Russia and China, can be said to represent the future of warfare, at least in the near- to mid- term. As a form of warfare which is not limited to any one domain, hybrid warfare is examined in its land, air, and maritime iterations by analysing the cases of the 2006 Lebanon War, the 2014 Ukraine crisis, and the South China Sea disputes. Its combination of opaqueness, effectiveness, impact, and strategic surprise makes it a carefully balanced and finely calibrated tool of international coercion.