For Bayinnaung, the work was not yet done. On 15 October 1569, Bayinnaung at Ayutthaya led several regiments of his wounded army to invade Lan Xang via Phitsanulok. Setthathirath again fled Vientiane. Bayinnaung's men pursued but could not find him. Tired by long marches in a mountainous country, the Burmese army gave up and left Vientiane in May 1570. Another expedition in 1572 after Setthathirath's death also failed to bring order. Finally, the king himself led another expedition, and put his nominee on the Lan Xang throne in November 1574. Aside from a minor rebellion in 1579, Lan Xang gave no trouble for the rest of Bayinnaung's reign.
Question: When was the first expedition?
Answer: 15 October 1569