1. Illusion of invulnerability. A feeling of power and authority is important to any decision-making group. It gives members confidence that they will be able to carry through on any decisions reached. However, if they come to believe that every decision they reach will automatically be successful, then they become prey to an illusion of invulnerability. Janis showed that American military leaders had this illusion in choosing not to fortify Pearl Harbor more heavily prior to the disastrous attack by the Japanese that led to U.S. entry into World War II.