Short answers, pulled from the story.
The law of war is the part of international law that regulates the conditions for initiating war, known as jus ad bellum, and the conduct of hostilities, known as jus in bello. It defines sovereignty, nationhood, occupation, the treatment of prisoners of war, and the prohibition of certain weapons that cause unnecessary suffering.
The five most commonly cited principles of international humanitarian law are military necessity, distinction, proportionality, humanity, and honor. Distinction requires separating combatants from civilians, while proportionality requires that harm to civilians not be excessive in relation to the direct military advantage expected.
The first traces of a law of war come from the Babylonians through the Code of Hammurabi, written around 1750 B.C. Later sources include the Book of Deuteronomy, the battlefield rules of the caliph Abu Bakr in the early 7th century, Augustine of Hippo's just war doctrine, and the Law of the Innocents given by Adomnan of Iona in 697.
The modern law of war is made up from three principal sources: lawmaking treaties or conventions, custom as articulated by the Martens Clause, and general principles such as distinction, proportionality, and necessity. Customary law is established by the general practice of nations together with their acceptance that the practice is required by law.
Historian Geoffrey Best called the period from 1856 to 1909 the law of war's epoch of highest repute. Its defining feature was the establishment by states of a written positive legal foundation that superseded a regime based mainly on religion, chivalry, and custom.
Under the 1949 Geneva Conventions, it is prohibited to fire on a person bearing a white flag, which indicates an intent to surrender or communicate. Captured unlawful combatants may lose prisoner of war protections only after a competent tribunal finds them ineligible, but they must still be treated with humanity and given a fair and regular trial.
Protocol I additional to the 1949 Geneva Conventions prohibits attacking persons parachuting from an aircraft in distress, regardless of the territory below. This protection does not apply to airborne troops, special forces, commandos, spies, saboteurs, liaison officers, and intelligence agents, who remain legitimate targets.