Cuba Operation Northwoods; was supposed to be a false flag operation against the Cuban government to have a reason to invade them.
However, rejected by J.F.K (I think it was one of the reason he was assassinated).
I believe that the method was used against middle eastern countries to gain natural resources and oil; Afghanistan (Gold, Diamond etc), Iraq (Oil).
Most of the people involed in the 9/11 attack was Saudi Arabian citizens, also making up that the Iraqi government possessed chemical weapons, to have another reason to invade them.