[6] In the midst of the disagreement, the Burmese invaded northern Ayutthaya with the assistance of Maha Thammaracha the royal viceroy and governor of Phitsanulok.

It was only then in 1564 that King Chakkraphat sent Princess Thepkasattri to Lan Xang along with a massive dowry in an attempt to buy back the broken alliance.

Facing the threat of a superior Burmese force, King Chakkraphat had lost a potential alliance with Lan Xang, the northern territories of Ayutthaya and his daughter.

Facing disease, malnutrition and demoralizing guerrilla warfare, King Bayinnaung was forced to retreat in 1565 leaving Lan Xang the only remaining independent Tai kingdom.

The plan would involve an overland invasion from Lan Xang with assistance from the royal navy in Ayutthaya passing up the Nan River.

Realizing Phitsanulok was too fortified, Setthathirath withdrew his attack, but set up a devastating counter ambush on his retreat to Vientiane in which five pursuing Burmese generals were killed.

Seizing on the weakness, King Chakkraphat ordered a second attack on Phitsanulok in which he successfully took the city, but could only briefly hold it having suffered repeated heavy losses.

[13] After a two-day struggle the Lan Xang forces prevailed at the Pa Sak Valley near Phetchabun, at which point one of the commanding generals from Nakhon Phanom broke south toward Ayutthaya.

The Burmese took several weeks to regroup and rest having taken Ayutthaya, which allowed Setthathirath to rally his forces and plan for prolonged guerrilla warfare.

Defeating the Khmer would have greatly strengthened Lan Xang, giving it vital sea access, trade opportunities, and most importantly, European firearms which had been growing use since the early 1500s.

The Khmer Chronicles record that armies from Lan Xang invaded in 1571 and 1572, during the second invasion King Barom Reacha I was slain in an elephant duel.

In 1571, a conspiracy between Lord Phya Nakhon and the former abbot of Wat Maximavat, who held personal grudges against Setthathirath, led to the king's murder in the southern frontier of the country.

Because Setthathirath left only a toddler as his heir, prince Noi Hno Muang Keo Koumane, the child's maternal grandfather, a military commander of common birth named Sen Soulintha, declared himself king.

With the country in chaos, Prince Noi Hno Muang Keo Koumane was always recognised as the rightful king by the people of Laos who campaigned for his return for many years.