Despite the 1962 accords of a second Geneva conference, Laos was being drawn increasingly into the Second Indochina War , as North Vietnam and the United States undermined the country's neutrality in the pursuit of their agendas in Vietnam.

Lao territory was a crucial part of the North Vietnamese war effort. They needed to control the mountainous eastern corridor of southern Laos in order to move soldiers and supplies to South Vietnam along the Ho Chi Minh Trail . The US saw no option but to challenge North Vietnam's strategy. So the right-wing Lao, the Americans and the Thais on the one side and the Pathet Lao, the North Vietnamese and their Chinese and Soviet backers on the other all tacitly agreed to pretend to abide by the accords, "guaranteeing Laos's neutrality" while in reality keeping the country at war.

In 1964, a new phase of the war in Laos began. With the US pushing hard for an escalation of the bombing , Souvannaphouma (kept in power with help from the US) gave the go-ahead for so-called "armed reconnaissance" flights over Laos, which essentially meant the US could bomb wherever it pleased.

The war took place in total secrecy . US ground troops were kept out and military planes had to take off outside the country. As journalist Christopher Robbins wrote, "There was another war even nastier than the one in Vietnam, and so secret that the location of the country in which it was being fought was classified." From 1964 until the ceasefire of February 1973, United States planes flew 580,944 sorties - or 177 a day - over Laos and dropped 2,093,100 tons of bombs - equivalent to one planeload of bombs every eight minutes around the clock for nine years - making Laos the most heavily bombed country per capita in the history of warfare.

When Nixon became US president in 1969, he initiated a policy of "Vietnamization" in which South Vietnam troops would gradually replace US ground forces, backed up by further escalation of the air war. This proved disastrous two years later, for operation Lam Son 719 , in which South Vietnamese troops invaded Laos in an attempt to sever the Ho Chi Minh Trail near Xepon in the south, the largest remaining communist stronghold after Cambodia. Five thousand South Vietnamese were killed or wounded, and more than one hundred US army helicopters shot down.