Chapter II: "Civilians and Civilian Population" of Protocol I Additional to the Geneva Conventions covers distinction. Article 50 defines who is a civilian and what is a civilian population. Article 51 describes the potection which should be given to civilian populations. Chapter III regulates the targeting of civilian objects. Article 8(2)(b)(i) Rome Statute of the International Criminal Court also prohibits attack directed against civilians. Not all states have ratified Protocol I or the Rome Statute, but it is an accepted principle of international humanitarian law that the direct targeting of civilians is a breach of the customary laws of war and is binding on all belligerents.
Luis Moreno-Ocampo Chief Prosecutor at the International Criminal Court investigated allegations of War Crimes during 2003 invasion of Iraq and he published an open letter containing his findings. In a section titled "Allegations concerning War Crimes" elucidates this use of distinction: