A false flag is, essentially, a misrepresentation of allegiance in an action of any significance (typically national, military, or political, but it can also apply on smaller scales such as social media) – leading to the appearance that an operation was carried out in support of a particular cause (an organization, individual, ideology, etc.) in order to discredit that cause. It is a form of smear tactic.
The sinking of the USS Maine (1898), later determined to have been accidental, was used by the mainstream media of the time -- i.e. newspapers owned by Hearst and Pulitzer -- to prevent a peaceful resolution of existing disputes with Spain, in spite of the lack of clear evidence, and is generally seen as being a precipitating cause of the Spanish-American War which began two months later. (This is where the cry "Remember the Maine!" comes from.) The papers essentially painted a false Spanish flag over an incident whose causes were not yet known.
1933: While responsibility for the Reichstag Fire has never been conclusively placed, there does not appear to be any evidence of the vast Communist plot for which the incident was used as evidence and thence justification for suspending key parts of the Weimar Constitution, which in turn allowed the arrest of the Communist delegates in the German Parliament, leaving the Nazi Party a majority.
1939: Nazi operatives attacked the radio tower at Gleiwitz (at that time a German town) while posing as Polish nationals -- creating the appearance of Polish aggression against Germany and thereby successfully drumming up popular support for the Nazi invasion of Poland.
1962: Operation Northwoods (1962) was a CIA plan to blame Cuba for a terrorist attack as a pretext for a "justified war" against them. A number of implementations were suggested, but all were rejected by President Kennedy. The existence of these plans only became public knowledge in 1997.
~2011: "Black bloc" provocateurs break shop windows during Occupy protests in various cities, leading newspapers to report or imply that the Occupy protesters were violent and destructive, and also discrediting black bloc tactics (such tactics having often been used for good aims).
(Possibly 2017: Black bloc actors again pose as anti-fascists while using fascistic tactics (attacking without provocation) at a small right-wing protest in Berkeley, CA. It appears that although most of the BB were in fact antifa legitimately chasing known fascists out of town, at least one or two of them were right-wing provocateurs; it's not clear yet who did what to whom.)
In the years after the 9/11 attacks, the term false flag has been increasingly used to refer to any deadly event, not necessarily directly perpetrated by the "flagging" government but possibly merely encouraged or permitted by them, and then carefully blamed on a particular target (government or other threat) so as to build domestic support for a war on the blamed party. Issuepedia uses the term engineered provocation for this meaning, as there may or may not be an actual "false flag" involved.
- Conservapedia: stub article as of 2018-08-30
- RationalWiki: redirects to "False flag operation" as of 2018-08-30
dKosopediano article as of 2008-01-10 SourceWatchno article as of 2018-08-30