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.