A private army (or private military) is a military or paramilitary force consisting of armed combatants who owe their allegiance to a private person, group, or organization, rather than a nation or state.
Private armies may form when land owners arm household retainers for the protection of self and property in times of strife and where and when central government is weak. Such private armies existed for example in the Roman Empire following the collapse of central authority. The dynamics at play in such circumstances can be observed in modern-day Colombia: on the one hand there are those forces affiliated with the drug cartels, existing to protect their criminality, and on the other those of the landlords created to resist kidnappings and extortion, i.e. the Autodefensas Unidas de Colombia.
In many places these private household retainers evolved into feudal like structures, formalising obligations and allegiances and becoming household troops, and in some cases gaining the strength to allow them to usurp power from their nominal suzerain or create new sovereign states.
Private armies may also form when co-religionists band together to defend themselves from real and perceived persecution and to further their creed, for example the Hussites, Mormon Nauvoo Legion and the Mahdi Army in Iraq; because of their nature such militias are formed by or fall under the influence of charismatic leaders and can become instruments of personal ambition.
- Sōhei: the warrior monks of Japan owed their loyalty not to the state or even the Emperor but to their monasteries.
- Crusading orders: for example the Knights Templar, Knights Hospitaller and the Teutonic Knights.
- Victual Brothers, a pirate brotherhood that for a time became a power in the Baltic.
- The Presidency armies of the British East India Company, and the militaries of the Dutch East India Company. Both possessed powerful fleets and created company run states many times the size of their homelands.
- The militaries of the Indian Princely States, under the British Raj, which were mainly for ceremonial duties, protection of their princes, and internal security within their states.
- Atholl Highlanders, modern Europe's only legal private army, now purely ceremonial.
- Dnipro Battalion, founded and controlled by Ukrainian oligarch Ihor Kolomoisky.
- German Freikorps after World War I were usually only loyal to their commanders, and not to the Weimar Republic.
- The Mongoose Gang was a private army or militia which operated from 1967 to 1979 under the control of Sir Eric Gairy, the Premier and later Prime Minister of Grenada.
- Regular military forces whose allegiance is to one person or group.
- the armed branch of a political party within a nation
- a paramilitary force
- a parapolice
- Irregular military forces: such as
- Range war
- Violent non-state actor
- "The Duke of Atholl". The Daily Telegraph. 17 May 2012. Retrieved 19 October 2012.
- "Meet the private army controlled by sacked Ukrainian billionaire Igor Kolomoisky". Business Insider. 25 March 2015. Retrieved 19 April 2015.