- 1a (1) : a state of usually open and declared armed hostile conflict between states or nations (2) : a period of such armed conflict (3) : state of war
- b : the art or science of warfare
- c (1) obsolete : weapons and equipment for war (2) archaic : soldiers armed and equipped for war
- 2a : a state of hostility, conflict, or antagonism
- b : a struggle or competition between opposing forces or for a particular end <a class war> <a war against disease>
War is an organised and often prolonged armed conflict that is carried out by states and/or non-state actors. It is characterised by extreme violence, social disruption, and economic destruction. 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 or intervention. The set of techniques used by a group to carry out war is known as warfare. An absence of war is usually called peace.
In 2003, Nobel Laureate Richard E. Smalley identified war as the sixth (of ten) biggest problem 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 nature, 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 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.
The deadliest war of the 21st century, in terms of cumulative number of deaths since start, is the Second Congo War, with 3–5 million deaths since 1998, although it was nearly invisible in non-African media. As of 2013, the largest ongoing conflicts in terms of deaths are the Mexican Drug War and the Syrian civil war.