A weapon is a tool used to apply or threaten to apply force for the purpose of hunting, attack or defense in combat, subduing enemy personnel, or to destroy enemy weapons, equipment and defensive structures. A weapon is therefore a device that changes the direction or magnitude of a force. In general, they can be defined as the simplest mechanisms that use mechanical advantage (also called leverage) to multiply force.

In attack, weapons may be used to threaten by direct contact or by use of projectiles. Weapons can be as simple as a club, or as complex as an intercontinental ballistic missile. Metaphorically speaking, anything capable of causing damage, even psychologically, can be referred to as a weapon. More recently, non-lethal weapons have been developed for para-military, security and even combat use, designed to incapacitate personnel and reduce collateral damage to property and environment.

Prehistoric weapons

Very simple weapon use has been seen in some communities of chimpanzees.Jill D. Pruetz1 and Paco Bertolani, Savanna Chimpanzees, Pan troglodytes verus, Hunt with Tools", Current Biology, March 6, 2007 Chimps in various parts of the world have been observed throwing rocks or stones as a means of attack. Craig Stanford, a primatologist and professor of anthropology at the University of Southern California, has suggested that the discovery of "spear" (essentially sharpened stick) use by chimpanzees probably means that early hominids used wooden spears as well, perhaps five million years ago.

Clubs and staves must have been used by early hominids, then, as they were nearly identical to spears, save the length (in clubs) and point (in both clubs and staves). In fact a tree branch, which could be considered a crude club or staff, must have been used to fashion a spear. Clubs themseves were also probably thrown, making the use of throwing sticks and a number of their various subtypes evident. Rocks were also thrown as early projectile weapons.

These earliest and most primitive weapons were the by-products of early human hunting – claws, teeth and horns of hunted animals, shaped or adapted for use as weapons. Stone axes were used as weapons very early in human history as personal weapons of direct attack, and as a particular type of simple tool that made up for comparative lack of natural weapons, such as claws, horns and teeth, in the human physiology.

The first human use of weapons is not easy to date, as these would probably have been wooden clubs, spears and unshaped stones thrown at prey or enemy—and none of these would leave an unambiguous record.

The earliest examples found are a cache of eight wooden throwing spears, the Schöninger Speere, which have been dated as 400,000 years ago.

By 250,000 years ago wooden spears were made with fire-hardened points. From 80,000 years ago humans began to make complex stone blades, which were used as spear points.