a series of central intelligence agency-backed intrusions from military region 3 (mr 3) and military region 4 (mr 4) of laos during 1969 and 1970 failed to interdict the trail. the massive operation lam son 719 assault staged in february 1971 failed to cut it. nevertheless, central intelligence agency (cia) backed laotian guerrillas and royal lao army (rla) military irregulars made further attempts at severing the communist logistical lines of communication.