Collaborationism is cooperation with the enemy against one's country of citizenship in wartime. The term is most often used to describe the cooperation of civilians with the occupying Axis Powers, especially Nazi Germany, Fascist Italy, and Imperial Japan, during World War II. Motivations for collaboration by citizens and organizations included nationalism, ethnic hatred, anti-communism, antisemitism, opportunism, self-defense, or often a combination of these factors. Some collaborators in World War II committed war crimes, crimes against humanity, or atrocities such as the Holocaust. More often collaborators simply "went along to get along," attempting to benefit from the occupation or simply survive. The definition of collaborationism is imprecise and subject to interpretation.
Stanley Hoffmann subdivided collaboration into involuntary (reluctant recognition of necessity) and voluntary (an attempt to exploit necessity). According to him, collaborationism can be either servile or ideological. Servile is service to an enemy based on necessity for personal survival or comfort, whereas ideological is advocacy for cooperation with an enemy power.  In contrast, Bertram Gordon used the terms "collaborator" and "collaborationist" for non-ideological and ideological collaborations, respectively. James Mace Ward has asserted that, while collaboration is often equated with treason, there was "legitimate collaboration" between civilian internees (mostly Americans) in the Philippines and their Japanese captors for mutual benefit and to enhance the possibilities of the internees to survive.Collaboration with the Axis Powers in Europe and Asia existed in varying degrees in all the occupied countries. Although the United Kingdom and the United States were never occupied, a British dependency, the Channel Islands near France, was under German occupation and thousands of American civilians in Asia were interned by Japan.
With the defeat of the Axis, collaborators were often punished by public humiliation, imprisonment, and execution. In France, 10,500 collaborators are estimated to have been executed, some after legal proceedings, others extrajudicially.
The opposite of collaborationism in World War II was "resistance", a term which also has a broad range of meaning and interpretations.
The term collaborate dates from 1871, and is a back-formation from collaborator (1802), from the French collaborateur as used during the Napoleonic Wars against smugglers trading with England and assisting in the escape of monarchists, and is itself derived from the Latin collaboratus, past participle of collaborare "work with", from com- "with" + labore "to work". The meaning of "traitorous cooperation with the enemy" dates from 1940, originally in reference to the Vichy Government of France which cooperated with the Germans, 1940-44.
John Hickman identifies thirteen reasons why occupied populations might hold collaborators in contempt.
During World War II, collaborationism existed to varying degrees in German-occupied zones.
In France, a distinction emerged between the collaborateur (collaborator) and the collaborationniste (collaborationist). The term collaborationist is mainly used to describe individuals enrolled in pseudo-Nazi parties, often based in Paris, who had belief in fascist ideology or were anti-communists.Collaborators on the other hand, engaged in collaboration for pragmatic reasons, such as carrying out the orders of the occupiers to maintain public order (policeman) or normal government functions (civil servants); commerce (including sex workers and other women who had relationships with Germans and were called, "horizontal collaborators"); or to fulfill personal ambitions and greed. Collaborators were not necessarily believers in fascism or pro-Nazi Germany.
Recent research by the British historian Simon Kitson has shown that French authorities did not wait until the Liberation to begin pursuing collaborationists. The Vichy government, itself heavily engaged in collaboration, arrested around 2,000 individuals on charges of passing information to the Germans. Their reasons for doing so was to centralise collaboration to ensure that the state maintained a monopoly in Franco-German relations and to defend sovereignty so that they could negotiate from a position of strength. It was among the many compromises that the government engaged along the way.
In Belgium, collaborators were organized into the VNV party and the DeVlag movement in Flanders, and into the Rexist movement in Wallonia. There was an active collaboration movement in the Netherlands.
Vidkun Quisling (1887-1945), a major in the Norwegian Army and former minister of defence, served the Nazis as prime minister. He gave his name to the high-profile government collaborator, now known as a Quisling.
After the German invasion of Greece, a Nazi-held government was put in place. All three quisling prime ministers, (Georgios Tsolakoglou, Konstantinos Logothetopoulos and Ioannis Rallis), cooperated with the Axis authorities. Small but active Greek National-Socialist parties, like the Greek National Socialist Party, or openly anti-semitic organisations, like the National Union of Greece, helped German authorities fight the Resistance, and identify and deport Greek Jews.
During the last two years of the occupation, the last quisling prime-minister, Ioannis Rallis, created the Security Battalions which were military corps that collaborated openly with the Germans, and had strong anti-communist ideology. The Security Battalions, along with various far-right and royalist organizations, and parts of the country's police forces of that era, were directly or indirectly responsible for the brutal killing of thousands of Greeks during the occupation. Contrary to what happened to other European countries, the members of these corps were never tried or punished for their crimes, due to the Dekemvriana events that erupted immediately after the liberation, followed by the White Terror and the Greek Civil War, two years later.
The main collaborationist regime in Yugoslavia was the Independent State of Croatia, a puppet state semi-independent of Nazi Germany. Leon Rupnik (1880-1946) was a Slovene general who collaborated as he took control of the semi-independent region of the Italian-occupied southern Slovenia known as the Province of Ljubljana, which came under German control in 1943. The main collaborationists in East Yugoslavia were the axis-puppet Serbian Government of National Salvation and the Chetniks, who collaborated with the Axis after 1941.
German citizen and non-Nazi Franz Oppenhoff accepted appointment as Mayor of the German city of Aachen in 1944, under authority of the Allied military command. He was assassinated on orders from Heinrich Himmler in 1945.
More recent examples of collaboration, according to some, have included institutions and individuals in Afghanistan who collaborated with the Soviet occupation until 1989 and individuals in Iraq and Afghanistan today who continue to work with invading American forces. In 2014 during the occupation of Crimea and ongoing War in Donbass, some Ukrainian citizens collaborated with the invading Russian forces.
In Palestinian society, collaboration with Israel is viewed as a serious offence and social stain and is sometimes punished (judicially or extrajudicially) by death. In addition, during the period of 2007-2009, around 30 Palestinians have been sentenced to death in court on collaboration-related charges, although the sentences have not been carried out.
In June 2009, Raed Sualha, a 15-year-old Palestinian boy, was brutally tortured and hanged by his family because they suspected him of collaborating with Israel. Authorities of the Palestinian territories launched an investigation into the case and arrested the perpetrators. Police said it was unlikely that such a young boy would have been recruited as an informer.
Governments, non-state actors, and private individuals cooperated and gave assistance to the Islamic State of Iraq and the Levant (ISIL or ISIS) during the Syrian Civil War, Iraqi Civil War, and Libyan Civil War.
In some colonial or occupation conflicts, soldiers of native origin were seen as collaborationist. This could be the case of mamluks and janissaries in the Ottoman Empire. In some cases, the meaning was not disrespectful at the beginning, but changed with later use when borrowed: the Ottoman term for the sipahi soldiers became sepoy in British India, which in turn was adapted as cipayo in Spanish or zipaio in Basque with a more overtly pejorative meaning of "mercenary".