War is an organized, armed, and often a prolonged conflict that is carried on between states, nations, or other parties typified by extreme aggression, social disruption, and usually high mortality. War should be understood as an actual, intentional and widespread armed conflict between political communities, and therefore is defined as a form of political violence. The set of techniques used by a group to carry out war is known as warfare. An absence of war (and other violence) is usually called peace.

In 2003, Nobel Laureate Richard E. Smalley identified war as the sixth (of ten) biggest problems facing the society of mankind for the next fifty years. In the 1832 treatise On War, Prussian military general and theoretician Carl von Clausewitz defined war as follows: "War is thus an act of force to compel our enemy to do our will."

While some scholars see warfare as an inescapable and integral aspect of human culture, others argue that it is only inevitable under certain socio-cultural or ecological circumstances. Some scholars argue that the practice of war is not linked to any single type of political organization or society. Rather, as discussed by John Keegan in his History of Warfare, war is a universal phenomenon whose form and scope is defined by the society that wages it. Another argument suggests that since there are human societies in which warfare does not exist, humans may not be naturally disposed for warfare, which emerges under particular circumstances. The ever changing technologies and potentials of war extend along a historical continuum. At the one end lies the endemic warfare of the Paleolithic^{[citation needed]} with its stones and clubs, and the naturally limited loss of life associated with the use of such weapons. Found at the other end of this continuum is nuclear warfare, along with the recently developed possible outcome of its use, namely the potential risk of the complete extinction of the human species.

In historical legal systems, an outlaw is declared as outside the protection of the law. In pre-modern societies, this takes the burden of active prosecution of a criminal from the authorities. Instead, the criminal is withdrawn all legal protection, so that anyone is legally empowered to persecute or kill them. Outlawry was thus one of the harshest penalties in the legal system, since the outlaw had only himself to protect himself, but it also required no enforcement on the part of the justice system. In early Germanic law, the death penalty is conspicuously absent, and outlawing is the most extreme punishment, presumably amounting to a death sentence in practice.

The concept is known from Roman law, as the status of homo sacer, and persisted throughout the Middle Ages. It was only in the modern period that the principle of habeas corpus was established, requiring that criminals must be judged in person by a court of law before they can legally be punished.

In the common law of England, a "Writ of Outlawry" made the pronouncement Caput gerat lupinum ("Let his be a wolf's head," literally "May he bear a wolfish head") with respect to its subject, using "head" to refer to the entire person (cf. "per capita") and equating that person with a wolf in the eyes of the law: Not only was the subject deprived of all legal rights of the law being outside of the "law", but others could kill him on sight as if he were a wolf or other wild animal.