In chapters 1 and 2 we have sketched out what post-1945 ISS looks like as a sub-field of IR, and surveyed the key debates and approaches that have determined the shape and content of the subject. We have addressed our central theme of evolution by identifying a branching out from narrow, largely state-centric and military–political conceptions of the subject to a much more diverse set of understandings which are often in contestation with each other. In this chapter we look separately at the driving forces behind the evolution of ISS. Why was it that different conceptions of the scope, referent objects and epistemological understandings of ISS emerged when they did? Why, indeed, did ISS coalesce as a distinct subject and why did it thereafter evolve as it did? Why has there been so much change and turbulence within this sub-field when its Realist underpinnings, with their emphasis on the permanence of the military threat in world politics suggests that there should be a lot of continuity?

As we will show in later chapters, there are some significant continuities in the ISS literature, but there are also many substantial changes. Sometimes the priority of a topic declines (as with arms control and deterrence towards the end of the 1980s), and sometimes the direction changes when wholly new topics become part of ongoing debates (as with economic, environmental, societal and human security). Sometimes the content or emphasis of ISS changes en bloc, but sometimes it evolves in different ways in different places.