The vast majority of work on the ethics of war focuses on traditional wars between states. In this chapter, I aim to show that this is an oversight worth rectifying. My strategy will be largely comparative, assessing whether certain claims often defended in discussions of interstate wars stand up in the context of civil conflicts, and whether there are principled moral differences between the two types of case. Firstly, I argue that thinking about intrastate wars can help us make progress on important theoretical debates in recent just war theory. Secondly, I consider whether certain kinds of civil wars are subject to a more demanding standard of just cause, compared to interstate wars of national-defence. Finally, I assess the extent to which having popular support is an independent requirement of permissible war, and whether this renders insurgencies harder to justify than wars fought by functioning states.