n. Warfare in which the combatants have markedly different military capabilities and the weaker side uses non-standard tactics such as terrorism.
2001
The conflict is an example of asymmetric warfare, which does not match the relatively even strength of two conventional military forces. It occurs when a weaker combatant uses nontraditional weapons and strategy in order to obtain a fighting advantage over a stronger opponent. The Palestinians are employing asymmetric tactics in order to achieve concessions from a conventionally stronger Israeli opponent, Cordesman says, attacking Israeli settlements, detonating car bombs, using the media as a 'political weapon,' and even hacking Israeli computers.
1995 (earliest)
Prof. T.V. Paul questions this explanation about why nations make war by raising the issue of 'asymmetric conflicts' — those initiated by so-called weaker powers against a country or coalition of superior military force. He cites six cases in the history of such asymmetric warfare between nations, of which one is the Indian-Pakistani war of 1965.
You also see the idea behind this phrase referred to as asymmetric conflict. This is a less popular term, but it's slightly older (first citation: 1993). It's used in the earliest citation for this sense of asymmetric warfare.