This thesis aims to investigate the link between a plurality of factors and events that led to the establishment of the Caliphate by Islamic State (IS) in 2014, examining the link between the phenomenon of imperialism and the ultimate rise of Islamic State. Given the devastation that has caused by terrorist activities, the source of terrorism and their grievances should be an urgent and pressing debate for states. Rather, the discourse has focussed on the prevention of terrorist attacks and the introduction of legislation to deal with terrorists and terror suspects, sometimes in contravention of human rights laws, as demonstrated by the war on terror. It is significant to note that the legacy of colonisation on terrorism is not widely considered in the theories about the origins of terrorism. The aim of this thesis is to examine mechanisms of control and exclusion that deprived the non-European world of 'Western' sovereignty and which continue to persist and endure in the international legal system, to the benefit of Western economic interests. Quasi- sovereignty, i.e. a lack of full sovereign rights over a state’s economic, political and cultural affairs, provides a very useful paradigm through which to develop a nuanced understanding of the ramifications of the violation of sovereignty in the Middle East. The assertion of the thesis is the system of law that emerged from the colonial experience has been instrumental in the formation of the international legal framework. The thesis begins with an examination of the rise of Islamic State by tracing the influence of Christianity and European culture on the evolution of international law, claiming that the modern international legal system is reflective of a framework that served to legitimise European colonial practices and interests, and while it operated universally, its meaning and application were dictated by those in power. Colonialism was therefore central to the formation of international law and the legacy of the governance of non-European people by European powers, that manifested in the League of Nations Mandate System and the ultimate creation of Iraq and Syria. The practices of “cultural subordination, and economic exploitation played an extraordinarily prominent role in shaping the relationship between international law and colonialism”.1 It is the argument of this thesis that these practices were not eradicated by decolonisation or the Mandate System, but continue to play an enduring and crucial role in international law, issues that are examined through the 1953 Iranian Coup, the 1991 Gulf War and the War on Terror. The Mandate ix System devised a set of legal structures and technologies that cemented and normalised the two-tier system of full- and quasi-sovereignty, denying equilibrium to non-Western states, issues that this thesis argues include multiple cause factors and form part of a plurality of events and issues from which the rise of Islamic State and the establishment of the Caliphate in Iraq and Syria was the consequence. As observed by Seumas Milne “the roots of the global crisis which erupted on September 11 lie in precisely the colonial experiences and the quasi-imperial system that succeeded them”.