sentences:  In World War II, targets found through cryptanalysis of radio communication were attacked only if there had been aerial reconnaissance in the area, or, in the case of the shootdown of Admiral Isoroku Yamamoto, where the sighting could be attributed to the Coastwatchers.During the Vietnam War, trucks attacked on the Ho Chi Minh trail were completely unaware of some sensors, such as the airborne Black Crow device that sensed their ignition.They could also have been spotted by a clandestine human patrol.