The second of two principles contained in the "Just War Theory," Jus Ad Bellum and Jus In Bello. Jus In Bellum in Latin means "The Law in Waging War." It defines standards by which a country can conduct war maintain that they have "just" actions in war. The term was coined in the same era in which the League of Nations originated. However, it was not used in doctrine until the late 1940s. Its principles include discrimination and proportionality.

Descrimination defines legitimate targets and proportionality defines how much force to be used.

Under the guidelines of Jus In Bello, the attack on Heroshimo, Japan by US forces with the use of the atomic bomb violates both the guidelines of descrimination and proportionality.

A group of principles intended as guidelines for the just prosecution of war whose origins go back at least as far as the writings of St. Thomas Aquinas (ca. 1225-1274 AD).

Includes the two principles of “discrimination” and “proportionality,” which attempt to define valid targets and acceptable applications of force, respectively.

Real understanding of the circumstances of the attacks on Hiroshima and Nagasaki would lead one to conclude that the use of the atomic bomb was actually fully justified under the guidelines of jus in bello.