by David Prentice, American Thinker:

The definition of a false flag is a hostile or harmful action (such as an attack) that is designed to look like it was perpetrated by someone other than the person or group responsible for it.

It was first done by pirates who would fly flags of friendly nations to allow themselves to get close enough to attack merchant ships. The historical use of a flag isn’t the key — it’s the deception.

This kind of tactic has been used often: in 1939, Hitler faked an attack by Poland on a German outpost so that he would have an excuse to invade Poland. A few years earlier Japanese forces set off a bomb next to a Japanese-owned rail line in Mukden as a pretext for the invasion of Manchuria.