This chapter introduces the law of war crimes including: its historic development, common legal issues such as identifying an armed conflict, and the specific offences constituting war crimes. As for the historic emergence of war crimes law, the chapter looks at the evolution of humanitarian law – on which war crimes law is based. The chapter discusses the key principles of humanitarian law (distinction, proportionality, etc), the challenges of regulating warfare, the emergence of individual criminal responsibility (i.e. war crimes), and the evolution of war crimes in non-international armed conflicts. As for ‘armed conflict’, the chapter discusses common challenges in identifying the character of the armed conflict (international or non-international), for example in situations of invitation, national liberation, proxy forces, and transnational conflict. The chapter reviews the requirements to qualify as an armed conflict – intensity of the conflict and organization of the parties. It then reviews specific illustrations of war crimes, including: crimes against non-combatants (killing, sexual violence, experiments, and so on); attacks on prohibited targets (for example, historic monuments, hospitals, and so on); and attacks likely to inflict disproportionate civilian harm compared to the anticipated military advantage; prohibited weapons; and prohibited methods of warfare.

Against weaker states and nonstate actors, powerful air forces working with competent proxy ground forces have been militarily effective in almost all cases, but their political effectiveness has varied by ambition and target. Air power was most likely to be politically effective in denial strategies against weak states but least effective when attempting to coerce nonstate actors by punishment. Air power was far more effective in breaking armies and toppling regimes than restoring political order in the aftermath of regime change. Some but not all of the strategies of the age of primacy will survive the transition to an age of great power rivalry. The persistence of small wars against nonstate actors will leave space for refinement of advisory models, close air support, and counter-network targeting. But missions that were feasible in an era of unchallenged air supremacy – CAS and persistent ISR – may become extinct against enemies possessing resilient IADS and long-range strike capabilities of their own.