"When you have eliminated the impossible,
whatever remains, however improbable, must be the truth." - Sir Arthur Conan Doyle
According to Wikipedia, the term False flag (or black flag) describes covert operations designed to deceive in such a way that the operations appear as though they are being carried out by entities, groups, or nations other than those who actually planned and executed them. Operations carried out during peace-time by civilian organizations, as well as covert government agencies, may by extension be called false flag operations if they seek to hide the real organization behind an operation. Geraint Hughes uses the term to refer to those acts carried out by "military or security force personnel, which are then blamed on terrorists."
The name "false flag" has its origins in naval warfare where the use of a flag other than the belligerent's true battle flag as a ruse de guerre, before engaging the enemy, has long been accepted. Such operations are also accepted in certain circumstances in land warfare, to deceive enemies in similar ways providing that the deception is not perfidious and all such deceptions are discarded before opening fire upon the enemy.
It turns out that nearly ALL modern day so-called "terrorist attacks" are really false flag, staged events, often utilizing crisis actors and hyper-reality training simulations, in order to sway public opinion towards supporting an agenda of the perpetrators, who are usually the government.