structural:expansion, quality:correction, structural:summarization, linguistic:voice_change
Siam (1563–1564)

With much of western mainland Southeast Asia under his control, Bayinnaung now felt ready to take on Siam. He needed an overwhelming advantage in manpower over Siam because unlike interior Shan states, Siam could not be overwhelmed by the strength of his Portuguese firearms alone. Siam was a prosperous coastal power in its own right, and its forces included Portuguese firearms, ships and mercenaries. On 16 July 1563, he sent an embassy to Siam, demanding one of the four white elephants in possession of the Siamese king as tribute. As expected, King Maha Chakkraphat duly refused. On 1 November 1563, five armies (60,000 men, 2400 horses and 360 elephants) left Pegu to start the campaign. Another army from Lan Na was supposed to come down but King Mekuti of Lan Na had revolted.

The invasion route was via central Siam. The armies took the key central town of Kamphaeng Phet on 4 December 1563. Three of the armies then fanned out to acquire the key central Siam cities of Sukhothai, Phitsanulok, and Sawankhalok. Aside from a five-day battle at Phitsanulok, the armies faced minimal opposition. The rulers of the three cities as well as the ruler of Phichit submitted, and were reappointed to their positions.

The armies then marched down to Ayutthaya. There, they were kept at bay for weeks by the Siamese fort, aided by three Portuguese warships and artillery batteries at the harbor. The invaders finally captured the Portuguese ships and batteries on 7 February 1564, after which the fort promptly fell. The Siamese king surrendered on 18 February 1564. Bayinnaung took all four white elephants, among other loot, and sent the fallen king to Pegu. He appointed Mahinthrathirat, a son of the fallen king, vassal king of Siam, and left a garrison of 3000. Thai sources state that Bayinnaung also took Prince Ramesuan (ราเมศวร), the eldest son of Maha Chakkraphat, back with him as well as Phraya Chakkri (พระยาจักรี), the chancellor of Ayutthaya, and Phra Sunthon Songkhram (พระสุนทรสงคราม), the military chief of Ayutthaya.