The term originated during World War II to describe rebels against Nazi Germany.
Historically, we find that people who are described as freedom fighters by supporters are called assassins, rebels or terrorists by their opponents. During the Cold War, the term freedom fighter was widely used by the United States and other Western Bloc countries to describe rebels in countries controlled by Communist governments or otherwise under the influence of the Soviet Union, including Hungary, Nicaragua, and Afghanistan.
The Soviet Union used the term in the same way, to describe rebel movements in countries controlled by or under the influence of the United States and other Western Bloc countries, such as Vietnam, Nicaragua, and Colombia. These rebels often used guerrilla tactics. Some were clearly assassins. But in general the asymmetric warfare employed on both sides made it difficult to assume anything but moral equivalence.
The term "freedom fighter", while indicating favor of some political group, often does not reflect any actual political position of those fighting. For example, to many people around the world, the leftist Sandinistas were freedom fighters. After their revolution took Nicaragua, the CIA funded a new opposition, the Contras, who were labeled as freedom fighters by the United States government and rebels or terrorists by the Sandinistas and Soviet Union. Of all political labels, "freedom fighter" is perhaps the most blunt term for "friend" - some think that it signals an unwillingness to abandon moral support regardless of methods, an unbreakable alliance between players.
The ambiguity of the term freedom makes the use of the label freedom fighter particularly useful for propaganda purposes. It is relatively simple to show that the "enemy" has done something which violates one of the many possible meanings of the word freedom, which allows the propagandist to appear to take the moral high ground by fighting for the cause of freedom. In addition to this, propagandists commonly use virtue words like freedom, which tend to evoke positive images in the target audience in order to attach those images and feelings to his cause.
Certain media agencies, notably the BBC, aside from attributed quotes, refuse to use the phrase "terrorist" or "freedom fighter", or even more descriptive and neutral terms such as "guerrilla" or "assassin", to avoid the political repercussions of the use of such words.
See guerrilla, assassin, terrorist, doublespeak.