Sort of hijacking (no pun intended) top comment since I've seen no mention yet of Operation Northwoods in this thread.

9/11 was Operation Northwoods.

Operation Northwoods was a proposed false flag operation against American citizens that originated within the U.S. Department of Defense (DoD) and the Joint Chiefs of Staff (JCS) of the United States government in 1962. The proposals called for the Central Intelligence Agency (CIA) or other U.S. government operatives to both stage and actually commit acts of terrorism against American military and civilian targets,[2] blaming them on the Cuban government, and using it to justify a war against Cuba. The possibilities detailed in the document included the possible assassination of Cuban immigrants, sinking boats of Cuban refugees on the high seas,[2] hijacking planes to be shot down or given the appearance of being shot down,[2] blowing up a U.S. ship, and orchestrating violent terrorism in U.S. cities.[3] The proposals were rejected by President John F. Kennedy.[4][5][6]

Fidel Castro had taken power in Cuba in 1959 and began allowing communists into the new Cuban government, nationalizing U.S. businesses and improving relations with the Soviet Union, arousing the concern of the U.S. military due to the Cold War. The operation proposed creating public support for a war against Cuba by blaming it for terrorist acts that would actually be perpetrated by the U.S. Government.[1] To this end, Operation Northwoods proposals recommended hijackings and bombings followed by the introduction of phony evidence that would implicate the Cuban government. It stated:

The desired result from the execution of this plan would be to place the United States in the apparent position of suffering defensible grievances from a rash and irresponsible government of Cuba and to develop an international image of a Cuban threat to peace in the Western Hemisphere.

Several other proposals were included within Operation Northwoods, including real or simulated actions against various U.S. military and civilian targets. The operation recommended developing a "Communist Cuban terror campaign in the Miami area, in other Florida cities and even in Washington", which involved the bombing of civilian targets themselves, which was to be blamed on the "irresponsible" Cuban government to paint a false image of Fidel Castro and misinform the American public.

The plan was drafted by the Joint Chiefs of Staff, signed by Chairman Lyman Lemnitzer and sent to the Secretary of Defense. Although part of the US government's anti-communist Cuban Project, Operation Northwoods was never officially accepted; it was authorized by the Joint Chiefs of Staff, but then rejected by President John F. Kennedy. None of the false flag operations became active under the auspices of the Operation Northwoods proposals. Similar operations were carried out inside Cuba as part of Operation Mongoose.

https://en.wikipedia.org/wiki/Operation_Northwoods