"False flag terrorism" occurs when elements within a government stage a secret operation whereby government forces pretend to be a targeted enemy while attacking their own forces or people. The attack is then falsely blamed on the enemy in order to justify going to war against that enemy. Or as Wikipedia defines it:
False flag operations are covert operations conducted by governments, corporations, or other organizations, which are designed to deceive the public in such a way that the operations appear as if they are being carried out by other entities. The name is derived from the military concept of flying false colors; that is, flying the flag of a country other than one's own. False flag operations are not limited to war and counter-insurgency operations, and have been used in peace-time; for example, during Italy's strategy of tension.