False flag
Similar false flag tactics were also employed during the Algerian civil war, starting in the middle of 1994. Death squads composed of Département du Renseignement et de la Sécurité (DRS) security forces disguised themselves as Islamist terrorists and committed false flag terror attacks. Such groups included the Organisation of Young Free Algerians (OJAL) or the Secret Organisation for the Safeguard of the Algerian Republic (OSSRA). According to Roger Faligot and Pascal Kropp (1999), the OJAL was reminiscent of "the Organization of the French Algerian Resistance (ORAF), a group of counter-terrorists created in December 1956 by the Direction de la surveillance du territoire (Territorial Surveillance Directorate, or DST) whose mission was to carry out terrorist attacks with the aim of quashing any hopes of political compromise".

Espionage: In espionage, the term "false flag" describes the recruiting of agents by operatives posing as representatives of a cause the prospective agents are sympathetic to, or even the agents' own government. For example, during the Cold War, several female West German civil servants were tricked into stealing classified documents by agents of the East German Stasi intelligence service pretending to be members of West German peace advocacy groups (the Stasi agents were also described as "Romeos", indicating that they also used their sex appeal to manipulate their targets, making this operation a combination of the false flag and "honey trap" techniques). 
According to ex-KGB defector Jack Barsky, "Many a right-wing radical had given information to the Soviets under a 'false flag', thinking they were working with a Western ally, such as Israel, when in fact their contact was a KGB operative."

Civilian usage: The term is popular amongst conspiracy theory promoters in referring to covert operations of various governments and claimed cabals. According to Columbia Journalism Review, this usage mostly "migrated to the right", however because some historical false flag incidents occurred, historians should not fully cede the usage of the term to conspiracy theorists. Perlman says "The real danger is if we use the nonattributive 'false flags' as shorthand for conspiracy theories, without explaining what they are and who is promoting them." At the same time, Perlman writes that "people yelling that any attack attributed to someone on 'their side' was committed by 'the other side' drown out the voices of reason."

Political campaigning: Political campaigning has a long history of this tactic in various forms, including in person, print media and electronically in recent years. This can involve when supporters of one candidate pose as supporters of another, or act as "straw men" for their preferred candidate to debate against. This can happen with or without the candidate's knowledge. The Canuck letter is an example of one candidate's creating a false document and attributing it as coming from another candidate in order to discredit that candidate.
In 2006, individuals practicing false flag behavior were discovered and "outed" in New Hampshire and New Jersey after blog comments claiming to be from supporters of a political candidate were traced to the IP address of paid staffers for that candidate's opponent.
On 19 February 2011, Indiana Deputy Prosecutor Carlos Lam sent a private email to Wisconsin Governor Scott Walker suggesting that he run a "'false flag' operation" to counter the protests against Walker's proposed restrictions on public employees' collective bargaining rights:

If you could employ an associate who pretends to be sympathetic to the unions' cause to physically attack you (or even use a firearm against you), you could discredit the unions ... Employing a false flag operation would assist in undercutting any support the media may be creating in favor of the unions.
The press had acquired a court order to access all of Walker's emails and Lam's email was exposed. At first, Lam vehemently denied it, but eventually admitted it and resigned.
Some conservative commentators suggested that pipe bombs that were sent to prominent Democrats prior to the 2018 mid-term elections were part of a false flag effort to discredit Republicans and supporters of then-President Donald Trump. Cesar Sayoc, motivated by his belief that Democrats were “evil”, was later convicted of mailing the devices to Trump's critics.
On the internet, a concern troll is a false flag pseudonym created by a user whose actual point of view is opposed to the one that the troll claims to hold. The concern troll posts in web forums devoted to its declared point of view and attempts to sway the group's actions or opinions while claiming to share their goals, but with professed "concerns". The goal is to sow fear, uncertainty, and doubt within the group often by appealing to outrage culture. This is a particular case of sockpuppeting and safe-baiting.

Ideological: Proponents of political or religious ideologies will sometimes use false flag tactics. This can be done to discredit or implicate rival groups, create the appearance of enemies when none exist, or create the illusion of organized and directed persecution. This can be used to gain attention and sympathy from outsiders, in particular the media, or to convince others within the group that their beliefs are under attack and in need of protection.
In retaliation for writing The Scandal of Scientology, some members of the Church of Scientology stole stationery from author Paulette Cooper's home and then used that stationery to forge bomb threats and have them mailed to a Scientology office. The Guardian's Office also had a plan for further operations to discredit Cooper known as Operation Freakout, but several Scientology operatives were arrested in a separate investigation and the plan was exposed.
According to PolitiFact, some false flag conspiracy theories (such as claims that mass shootings are hoaxes) are themselves spread by astroturfing, which is an attempt to create false impression of popularity in a belief.

See also: 

Concepts: Agent provocateur
Black propaganda
Casus belli
Denial and deception
Fear mongering
Frameup
Front organization
Joe job, a similar online concept
Mimicry
Red herring
State terrorism
Strategy of tension

References: 

External links:  The dictionary definition of false flag at Wiktionary