Is War Ever Justified?
Mishnah Sanhedrin 4:5
“Whoever destroys a soul, it is considered as if he destroyed an entire world. And whoever saves a life, it is considered as if he saved an entire world”. (Mishnah Sanhedrin 4:5; Yerushalmi Talmud 4:9)
Qur'an 5:32
“Whoever kills a soul… --it is as if he had slain mankind entirely. And whoever saves one--it is as if he had saved humankind entirely.”
1st Buddhist Precept
The first of the five precepts—Buddhist guidelines for an ethical life—is to refrain from taking life, often phrased as a vow not to kill living creatures. What qualifies as a living creature varies among Buddhist schools, but humans and all other animals are included.
From a Mahayana perspective, it is possible to kill without violating the first precept if the killing is of widespread benefit and the killer is motivated solely by compassion. A famous example of this utilitarian view in traditional texts is a story about the Buddha’s former life as a ship captain, found in the Upayakausalya, or Skill in Means sutra, in which he killed a murderous stowaway on his ship to save his five hundred-person crew from being murdered or from blindly killing one another in an attempt to stop the stowaway. Though rare circumstances like this may not be considered violations of the precept, they still create karmic consequences.
Christian Just War Theory
The basic assumption of modern Christians is that war is rarely justified and should be avoided unless the Just War conditions are met.
The four most important conditions are: (1) the war must be declared openly by a proper sovereign authority (e.g., a government, not an individual); (2) the war must have a just cause (e.g., defense of the common good or a response to grave injustice); (3) the warring state must have just intentions (i.e., it must wage the war for justice rather than for self-interest); and (4) the aim of the war must be the establishment of a just peace.
Since the end of World War II it has become customary to add three other conditions: (1) there must be a reasonable chance of success; (2) force must be used as a last resort; and (3) the expected benefits of war must outweigh its anticipated costs. (Encyclopedia Britannica)