The Ho Chi Minh Trail, a maze of roads, trails, and waterways in eastern Laos, provided the conduit for supplies and replacements from North Vietnam to its forces fighting in South Vietnam. The supplies and people were infiltrated through passes in the mountains separating Laos and North Vietnam onto the Trail, which led into South Vietnam through Laos and Cambodia. Transportation on the Trail grew from porters and bicycles to a fleet of trucks. Infiltration began early in the war, and the United States began to interdict these supplies and people from the air to assist the South Vietnamese, using a variety of aircraft and weapons. As the United States increased its efforts to stop the trucks, the North Vietnamese increased their efforts to bring down U.S. aircraft, using larger and larger antiaircraft guns and, ultimately, surface to air missiles. The most effective truck killers were the gunships, cargo aircraft equipped with guns firing from the left side of the aircraft and optical and electronic sensors to detect targets. The ultimate gunship was the AC-130E, which carried a 105-mm Army howitzer that fired a 5.6-pound projective and could operate above 10,000 feet.   In 1968, the United States began to operate a network of acoustic and seismic sensors in southern Laos that detected men and trucks transiting the Trail. Orbiting aircraft relayed signals from the sensors to a center in Thailand that analyzed the signals to determine numbers and location, then sent this information to controllers that dispatched attack aircraft. Sensors and methods improved over the years, increasing the accuracy of attacks. The invasion of South Vietnam in 1972 necessitated the use of most aircraft in direct support of combat troops there, effectively ending aerial interdiction of the Ho Chi Minh Trail in Laos.