Psychological warfare developed as a non-violent weapon meant to influence enemy soldiers and civilians through the use of paper leaflets or " paper bullets." Psy-war aims to demoralize a soldier, to weaken their resistance, or to convince a soldier to surrender to a stronger military force.

During World War II, the Psychological Warfare Branch (PWB), under the command of General Douglas MacArthur, operated in the southwest Pacific arena. The PWB used leaflets against Japanese soldiers occupying the Philippines and later towards Japanese soldiers and civilians in Japan. Selected military personnel appointed to the PWB received training in psychological warfare. J. Robert Sandberg and Frank M. Hallgren, both from Lincoln, Nebraska, were trained to work with the crews of airplane bombers and drop leaflets over target areas in the Philippines and Japan.

For more information, please take a look at the Sandberg-Hallgren Collection.