In military science, defense (or defence) is the art of preventing an enemy from conquering territory; usually via fortifications. Defense may also be a euphemism for war.

It is a widely accepted observation that the defending side in a conflict will require substantially fewer resources to win than does the offensive side. However, in situations where the attackers have a technological advantage over the defenders, as, for example, in various cases of colonial expansion, this can be sufficient to overcome the defenders' material resource advantages.

Military defense also refers to the use of weapons, such as artillery guns, tanks, planes, etc, in war or battle to defend a certain stronghold, be it country or town or fort. See also: Arms trade