A false flag is a type of covert operation that is performed either by the military or paramilitary groups. The operation is designed to appear if it is carried out by a group other than the one that actually did it.
Many believe that false flag attacks are carried out when a government wants to carry out an attack on its own soil. An example of a false flag attack was Operation Himmler. The Nazis faked attacks on their own people and blamed it on Poland. This false flag was carried out in order to justify the Nazi invasion of Poland.