Fighting in Laos resumed later in 1962 after the signing of the 1962 Declaration on the Neutrality of Laos in Geneva by fourteen nations including North Vietnam and the United States. An estimated 10,000 North Vietnamese were still present in Laos, despite the stipulation their government had signed at Geneva that withdrawal of all foreign troops be completed by October 7. Because the North Vietnamese did not respect the withdrawal requirement of the Geneva agreement, the United States stepped up military aid to the Kingdom of Laos but it avoided deploying ground troops which would have violated the agreement.
As part of the American effort to support the Kingdom of Laos, United States Central Intelligence Agency (CIA) personnel operating from a base at Udon Thani, Thailand, took over the support of 30,000 to 36,000 irregulars, including Hmong guerrillas who bore the brunt of the fighting in northern Laos. The irregulars, who became known as the Secret Army, were instrumental in helping to rescue a large number of United States airmen who were shot down over Laos. By this time, the Hmong leader Vang Pao had risen to the rank of general in the Royal Lao Army and commanded the Second Military Region.
In October 1964, in response to an offensive by the Pathet Lao and North Vietnamese to expel the Laotian Neutralists from the Plain of Jars, the United States began providing air support against Pathet Lao positions and North Vietnamese supply lines. However, it was not until March 1966 at Phoukout, northwest of the Plain of Jars, that the Pathet Lao started to win major battles against the Royal Lao Army. In July 1966, the Pathet Lao won another major battle in the Nambak Valley in northern Louangphrabang Province by overrunning a Royal Lao Army base and inflicting heavy casualties. These victories gave the Pathet Lao new momentum in the war for control of Laos.
Meanwhile, in southern Laos, where the North Vietnamese had been working steadily every dry season to expand the Ho Chi Minh Trail network leading into the Republic of Vietnam (South Vietnam), the intensity of the air war also grew. The air war in Laos operated under a complicated command and control system that involved the United States embassy in Vientiane, the Military Assistance Command Vietnam in Saigon, Royal Thai air bases in Thailand, the commander in chief Pacific in Honolulu, and sometimes even the White House.
During the June 1969 rainy season, the Pathet Lao and two North Vietnamese battalions, using Soviet tanks, pushed the Royal Lao Army and the Neutralists out of their base at Muang Souy northwest of the Plain of Jars. Fighting continued during the monsoon season. In September 1969, Vang Pao's Hmong, supported by American bombing, launched a series of surprise attacks against key points on the Plain of Jars. A new North Vietnamese army division joined the battle shortly thereafter and by February 1970 had regained all of the devastated plain.
In 1970, despite eight years of ground offensives by the Royal Lao Army and massive United States air support, the Pathet Lao had grown into an army of 48,000 troops and was prepared to challenge Royal Lao Army forces on their own territory by mounting large offensives in the south engaging an even greater number of North Vietnamese forces. The introduction of Soviet-made long-range 130mm artillery pieces onto the battlefield in that year allowed the Pathet Lao and North Vietnamese to neutralize to some extent the Royal Lao Army's advantage of air superiority.
By December 1971, the Pathet Lao had taken Paksong on the Bolovens Plateau and had invested the main Hmong base at Longtiang. A major North Vietnamese-Pathet Lao offensive against Vang Pao and the Hmong began in mid-December 1971 and lasted until the end of April 1972. This battle involved more than twenty North Vietnamese battalions and some 10,000 Hmong irregulars and Royal Lao Army defenders. Communist advances continued into 1972 and encircled Thakhek on the Mekong, and Vientiane.
The cease-fire of February 22, 1973, ended United States bombing and temporarily halted ground offensives. The Pathet Lao, however, used the cessation of military operations to resupply their forces over the long and exposed roads from North Vietnam.