Definition of false flag in US English:

false flag

noun

  • 1A flag flown to disguise the true identity or affiliation of a ship.

    ‘Captain Young hauled down his false flag, hoisted the American ensign, and opened fire’
    figurative ‘he hides behind the false flag of patriotism’
  • 2usually as modifier A political or military act orchestrated in such a way that it appears to have been carried out by a party that is not in fact responsible.

    ‘some believe that the bombings were a false flag operation by state forces’