criteria are termed "irregular forces". Types The term "irregular military" describes the "how" and "what", but it is more common to focus on the "why" as just about all irregular units were created to provide a tactical advantage to an existing military, whether it was privateer forces harassing shipping lanes against assorted New World colonies on behalf of their European contractors, or Auxiliaries, levies, civilian and other standing irregular troops that are used as more expendable supplements to assist costly trained soldiers. Bypassing the legitimate military and taking up arms is an extreme measure. The motivation for doing so is often used as the basis of the primary label for any irregular military. Different terms come into and out of fashion, based on political and emotional associations that develop. Here is a list of such terms, which is organized more or less from oldest to latest: Auxiliaries – foreign or allied troops supplementing the regular army, organized from provincial or tribal regions. In the Imperial Roman army, it became common to maintain a number of auxiliaries about equal to the legionaries. Levies – feudal peasants and freemen liable to be called up for short-term military duty. Privateer – a "for-profit" private person or ship authorized and sponsored by a government by letters of marque to attack foreign vessels during wartime and to destroy or disrupt logistics of the enemy during "peacetime", often on the open sea by attacking its merchant shipping, rather than engaging its combatants or enforcing a blockade against them. Revolutionary – someone part of a revolution, whether military or not. Guerrilla – someone who uses unconventional military tactics. The term tends to refer to groups engaged in open conflict, rather than underground resistance. It was coined during the Peninsula War in Spain against France. Montoneras – they were a type of irregular forces that were formed in the 19th century in Latin America. Franc-tireur – French irregular forces during the Franco-Prussian War. The term is also used in international legal cases as a synonym for unprivileged combatant (for example the Hostages Trial [1947–1948]). Militia – military force composed of ordinary citizens. Ordenanças – The Portuguese territorial militia system from the 16th century to the 19th century. From the 17th century, it became the third line of the Army, serving both as local defense force and as the mobilization system that provided conscripts for the first (Regular) and