Burmese–Siamese War (1568–1569)

In 1568, Ayutthaya king Maha Chakkraphat requested King Setthathirath of Lan Xang to attack Phitsanulok, ploying to arrest its King Mahathammarachathirat. When Mahathammarachathirat asked Ayutthaya for assistance Maha Chakkraphat dispatched general Phya Siharat–Dejo, tasking him with detaining Mahathammarachathirat. Siharat–Dejo instead remained in Phitsanulok and disclosed Maha Chakkraphat's true intentions, pushing Burma into an armed intervention.

A Burmese force broke through the Lao lines and joined the defenders of Phitsanulok which was besieged at the time. In the meantime, an Ayutthayan army advanced on Phitsanulok under the guise of reinforcements. Upon their arrival the Ayutthayans were asked to stay outside the city, on the same night the Phitsanulokans launched flaming rafts on the Ayutthayan fleet, destroying it. The losses prompted both the Lao and Ayutthayans to break the siege and retreat, the Lao troops later ambushed and annihilated a Burmese force that attempted to chase them.[11][12]

During the course of his retreat, Maha Chakkraphat unsuccessfully attacked Kamphaeng Phet. His plans changed however when he learned that was on an official visit to Burma, causing him to return to Phitsanulok. The Ayutthayans then proceeded to kidnap all of Mahathammarachathirat's family but his son Prince Naresuan who was accompanying his father. The kidnapping was to dissuade Phitsanulok from launching counter-attacks on Ayutthaya, nonetheless this act led the Burmese to initiate a joint invasion of Ayutthaya with the help of their Thai puppet states.[13]

Having gained the support of northern Thai states, Bayinnaung amassed five armies that consisted of 54,600 men, 5,300 horses, and 530 elephants according to Burmese sources.[2] Thai sources mention an army consisting of 546,000 infantrymen and 53,000 in cavalry, however, that was likely an exaggeration. The Burmese marched from the north until encountering a Lao army at the Pa Sak Valley near Phetchabun. The Lan Xang forces prevailed at which point one of the commanding generals from Nakhon Phanom broke south toward Ayutthaya. The Burmese rallied and were able to destroy the divided forces, and King Setthathirath had to retreat toward Vientiane.[14]

The Burmese then laid siege to Ayutthaya city. After heavy cannon fire put a stop to Burmese attempts at digging up to the walls, the invaders began building a bridge at Koh Keo in order to access the walls from a new direction. Maha Chakkraphat died during the course of the siege, therefore his son Mahinthrathirat ascended the throne. Frequent Ayutthaya sorties once again prevented the Burmese from finishing the construction. Bayinnaung then sent an Ayutthayan noble he held captive under the guise of a deserter. Not only was the spy allowed into the city but he was also put into a position of power. On the night of 7 August 1569, the spy opened the gates of the city, bringing its downfall. Mahinthrathirat along with his family and the nobility were captured and taken to Pegu. Mahinthrathirat died on the way in the same year.[15]^: 63 Ayutthaya became a Burmese vassal state, with Mahathammarachathirat appointed king.[14][4][16]

The Burmese took several weeks to regroup and rest having taken Ayutthaya, which allowed Lan Xang to rally their forces and plan for prolonged guerrilla warfare. The Burmese arrived in Vientiane and were able to take the lightly defended city. Setthathirath began a guerrilla campaign from his base near Nam Ngum, northeast of Vientiane. In 1570, after having no success against the Laotian guerrilla forces, Bayinnaung retreated and has chased by the forces of Setthathirath. The Laotians were able to capture 30,000 troops, 100 elephants, and 2,300 pieces of ivory from the Burmese.[16]