The Law of War

The Law of War is fascinating reading.  Source: http://ogc.osd.mil/images/law_war_manual_december_16.pdf

Excerpt:
3.5.1 General Distinction Between Jus in Bello and Jus ad Bellum. As a general matter, jus in bello and jus ad bellum address different legal issues and should not be conflated.

Conflating jus in bello and jus ad bellum risks misunderstanding and misapplying these concepts. For example, in jus ad bellum, proportionality refers to the principle that the overall goal of the State in resorting to war should not be outweighed by the harm that the war is expected to produce. However, the principle of proportionality in jus in bello generally refers to the obligations to take feasible precautions in planning and conducting attacks and to refrain from attacks in which the expected loss of civilian life, injury to civilians, and damage to civilian objects incidental to the attack would be excessive.  Therefore, although a jus ad bellum proportionality analysis might consider the harm suffered by enemy military forces in the fighting, a jus in bello proportionality analysis would not.

Note: Jus ad bellum deals with the question as to whether the war was justly started (i.e. for self-defense).  Jus in bello asks whether the war is being justly fought (i.e. avoiding civilian causalties where possible).