Communist forces broke the Laotian cease-fire of February 22, 1973 on March 27, 1975. North Vietnamese-Pathet Lao forces launched a strong attack against Vang Pao's Hmong defenders blocking the road junction linking Vientiane, Louangphrabang, and the Plain of Jars. Prince Souvanna Phouma, the prime minister of Royal Laos, wishing to avoid bloodshed, ordered Vang Pao only to defend himself and refused to allow air strikes in his support. The Pathet Lao singled out the Hmong as enemies to be shown no quarter. Realizing that the Hmong were being abandoned and the penalty they faced if left to the mercy of the Pathet Lao, Vang Pao and twelve Hmong leaders signed a treaty on May 10 agreeing to leave Laos and never return.
Meanwhile, a campaign of intimidation against rightist members of the Provisional Government of National Union (PGNU) and military officers gathered momentum in Vientiane. Demonstrators used inflation and other popular grievances to mobilize support for the eighteen-point program of the National Political Consultative Council. Prince Souvanna tried at first to ban the demonstrations but later gave in and sided with their aims. The May Day holiday provided the pretext for the largest demonstration to date, followed a week later by a demonstration against the rightist army and police. Four rightist ministers, including the defense minister, Sisouk na Champasak, fled. Another minister, Boun Om, was assassinated in the capital.
Elsewhere, takeovers of government offices and orchestrated demonstrations led to the entry of Pathet Lao troops into Pakxé, Savannakhét, Thakhek, and other towns during May "to secure their defense." People's revolutionary committees surfaced to seize administrative power from the remnants of the Royal Laos government. On August 23, the Pathet Lao completed its seizure of local power with the takeover of the Vientiane city administration by a revolutionary committee. The Pathet Lao announced that military units had requested Pathet Lao "advisers," thereby facilitating the integration of the army.
Kaysone Phomvihan, leader of the Pathet Lao, in a speech in Vieng Xay on October 12, declared that "the revolution will speed up." Simultaneously, the National Political Consultative Council (NPCC) established new screening procedures for candidates for election that effectively eliminated all those who had not supported the Lao Patriotic Front (Pathet Lao). In November, elections were held in the "new zone," the former Royal Laos government zone. Eligible voters were required to vote for a list of candidates whose names were distributed the evening before. Candidates were local party administrators, whose identities had been kept secret up to then. On November 28, demonstrators demanded the dissolution of the PGNU and the NPCC as inappropriate to the situation. The next day, Souvanna Phouma and Souphanouvong flew to Louangphrabang and persuaded the king to abdicate.