A proxy war is an armed conflict between two states or non-state actors which act on the instigation or on behalf of other parties that are not directly involved in the hostilities. In order for a conflict to be considered a proxy war, there must be a direct, long-term relationship between external actors and the belligerents involved. The aforementioned relationship usually takes the form of funding, military training, arms, or other forms of material assistance which assist a belligerent party in sustaining its war effort.
During classical antiquity and the Middle Ages, many non-state proxies were external parties which were introduced to an internal conflict and aligned themselves with a belligerent in order to gain influence and further their own interests in the region. Proxies could be introduced by an external or local power and most commonly took the form of irregular armies which were used to achieve their sponsor's goals in a contested region. Some medieval states such as the Byzantine Empire used proxy warfare as a foreign policy tool by deliberately cultivating intrigue among hostile rivals and then backing them when they went to war with each other. Other states regarded proxy wars as merely a useful extension of a preexisting conflict, such as France and England during the Hundred Years' War, both of which initiated a longstanding practice of supporting piracy which targeted the other's merchant shipping. The Ottoman Empire likewise used the Barbary pirates as proxies to harass Western European powers in the Mediterranean Sea.
Since the early twentieth century, proxy wars have most commonly taken the form of states assuming the role of sponsors to non-state proxies, essentially using them as fifth columns to undermine an adversarial power. This type of proxy warfare includes external support for a faction engaged in a civil war, terrorists, national liberation movements, and insurgent groups, or assistance to a national revolt against foreign occupation. For example, the British partly organized and instigated the Arab Revolt to undermine the Ottoman Empire during World War I. Many proxy wars began assuming a distinctive ideological dimension after the Spanish Civil War, which pitted the fascist political ideology of Italy and National Socialist ideology of Nazi Germany against the communist ideology of the Soviet Union without involving these states in open warfare with each other. Sponsors of both sides also used the Spanish conflict as a proving ground for their own weapons and battlefield tactics. During the Cold War, proxy warfare was motivated by fears that a conventional war between the United States and Soviet Union would result in nuclear holocaust, rendering the use of ideological proxies a safer way of exercising hostilities. The Soviet government found that supporting parties antagonistic to the US and Western nations was a cost-effective way to combat NATO influence in lieu of direct military engagement. In addition, the proliferation of televised media and its impact on public perception made the US public especially susceptible to war-weariness and skeptical of risking American life abroad. This encouraged the American practice of arming insurgent forces, such as the funneling of supplies to the mujahideen during the Soviet–Afghan War.