A private army (or private military) is a military force under the command of a private person or organization, rather than a nation or state.
History
Private armies may form when landowners 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. Muerte a Secuestradores and 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 to 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, the 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.
Examples
East Asia
Sōhei: the warrior monks of Japan owed their loyalty not to the state or even the Emperor but to their monasteries.
The militaries of the Indian princely states, during the British Raj, which were mainly for ceremonial duties, protection of their princes, and internal security within their states.
In the Philippines, the revolutionary Emilio Jacinto formed his own guerrilla force rather than join the Filipino Army. During World War II, some private armies such as the Bisigbakal ñg Tagala, Makapili, and Pambansang Pag-asa ng mga Anak ni Rizal were formed to support the Japanese. After the war, there were the Civilian Guards (Guardia Civil) formed by landowners in the Central Luzon to fight the Huks. After martial law was declared in 1972, the army broke up 149 private armies loyal to politicians in provincial areas. In 2001, the Ampatuan Militia was formed, and in 2009 it committed a massacre, which led to its disbandment in the same year. In 2013, militants claiming to be the Royal Forces of the Sultanate of Sulu tried to invade Sabah at the direction of a claimant to the throne.
Europe
Victual Brothers, a pirate brotherhood that for a time became a power in the Baltics.
Atholl Highlanders, a purely ceremonial group which claims to be the only private army in Europe.
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.
↑Gaaze, Konstantin. "Putin's National Guard Gambit". Moscow: Carnegie Moscow Center. He has redistributed power in the security sector and taken personal control of the part of it that may play a decisive role in the country in the next two years.