Furthermore, no modern army ever went into operation with certain units designated as wholly and exclusively defensive, and certain others as wholly and exclusively offensive. (The Great Wall of China is the world's most celebrated example of purely defensive planning, yet it protected Chinese offensive bases for twenty-one hundred years.) But in Psychological Warfare, the Japanese-language short-wave broadcasts from San Francisco had no imaginable effect on the American forces in the Pacific. The only people who could understand them were the Japanese-language officers in G-2 and ONI offices; their personal vexation did not matter. The offensive operations of combat troops are predicated upon finding the enemy, effecting contact, and either destroying the enemy or making him yield terrain. The defensive operations of combat troops, contrariwise, are planned with a view to resisting an enemy who has been met. In Psychological Warfare, operators and enemy do not effect contact. The audience cannot strike back through a radio set; the enemy reader cannot throw a leaflet back at the bomber which has dropped it on him. When American planes bombed German radio stations, they did not do so because the flight commander was trying to get German propaganda off the air; they did so because the Americans were trying to break up the entire German communications network. It is almost impossible to pinpoint radio transmitters and printing presses with such accuracy as to deny the enemy all chance of talking back. In a purely physical sense, there are only two sets of measures whereby an actual defense can be set up against Psychological Warfare. Each is a measure of desperation; neither is considered effective; the Americans did not bother with either in World War II. The first physical defense consists of radio jamming and of the planned interception of enemy leaflet raids. Radio jamming is ineffectual except in the case of an enemy possessing hopelessly inferior signal equipment. (The Japanese tried to jam our radio at Saipan, just as the Germans tried to jam BBC. They impeded reception, but they never succeeded in blocking it out altogether.) The second physical defense consists of destroying reception facilities. It is possible to sweep an occupied territory and to sequester _almost_ all the radio sets in use. It is possible to issue a military order that any soldier or civilian found in possession of enemy printed matter will be court-martialed and punished. These measures are useful to dictators having secret police, and among armies having the Prussian level of discipline, with the enlisted men regarded as robots. It is not to be expected that they would work against Americans. Therefore, propaganda does not meet propaganda. Combat forces _meet_; Psychological Warfare forces pass one another in opposite directions.