How can a widely hated, massively outnumbered and ludicrously outgunned organisation expands to occupy over 120 cities, towns and villages from the Southern Philippines to Western Libya? How can it endure and survive a military coalition of over 150 armed state and nonstate actors? How did ISIS/IS and their predecessors fight? And how can we account for their combat effectiveness? This book describes and analyses how ISIS/IS fights in Iraq, Syria, Libya and Egypt. It analyses the military-making of ISIS/IS and their predecessors. The analysis focuses on 17 urban battles in Fallujah, Mosul, Ramadi, Raqqa (City and Governorate), Derna, Sirte and Northeastern Sinai. The book is based on fieldwork, dozens of interviews with soldiers and fighters who engaged ISIS/IS and their predecessors, and hundreds of ISIS/IS combat-relevant publications, audio- and video-releases. The findings contribute to our understanding of insurgencies’ combat effectiveness and offer insights on how ISIS/IS, like-minded organisations, and other armed nonstate actors may or will fight in the future.