This chapter provides an overview of the birth and the military build-up of ISIS/IS in Syria, as of 9 April 2013. It aims to explain how ISIS/IS was able to gradually develop their combat capacities and the resulting combat and military effectiveness in Syria. The chapter then focuses on describing and analysing the battlefronts of Raqqa Governorate between 2013 and 2019. Raqqa City, the governorate’s provincial capital, was the first “capital” of the organisation. Arguably, ISIS/IS had shown its maximum combat capacities in Syria during its occupation of the governorate and in defence of its “capital.” The chapter is partly based on interviews with Syrian rebels and soldiers who fought against IS in eight Syrian governorates: Raqqa, Deir Ezzor, Aleppo, Homs, Hama, Latakia, Damascus, and Rif Dimashq. It is also based on documents, audio-visual and photographic releases produced by ISIS/IS in Syria. The chapter also relies on official documents released by the US government, Operation Inherent Resolve (OIR), and other open-source materials. The chapter is composed of five sections. It overviews the military build-up of ISIS/IS in Syria since its establishment in April 2013. It then outlines the details of the battlefronts of Raqqa Governorates within specific timeframes. It analyses how IS fight in Syria, using data and observations from the Raqqa battlefronts as well as others, such as Deir Ezzor and Aleppo Governorates. Finally, it reflects on the future of IS insurgency in Syria after losing Raqqa and other territories and shifting back to mainly guerrilla and terrorism tactics.