Initial invasion Bayinnaung, with his quick rise to power and influence, subsequently demanded two of King Chakkraphat's white elephants as tribute to the rising Toungoo Dynasty. Chakkraphat refused, leading to Burma's second invasion of the Ayutthaya Kingdom. Bayinnaung left Bago on 27 December 1563 and marched his force to Mottama. There, he organized his army into 5 divisions.
King Bayinnaung, familiar with the terrain of Siam due to his prior expeditions with the late
Tabinshwehti, entered Siam through what is now known as
Three Pagodas Pass in Kanchanaburi Province. A separate Burmese army entered through Mae Lamao Pass in what is now
Tak Province. The Siamese forces started the defense of Ayutthaya, with Phraya Chakri commanding 15,000 men to the Lumphli Fortress in the northern of Ayutthaya, Chao Phraya Mahasena commanding 10,000 men to the Ban Dokmai Han Tra Fortress in the east, Phraya Phra Khlang commanding 10,000 men to guard the south, and Phra Sunthorn Songkhram commanding 10,000 men to set up at Jampa Fortress in the west. However, the Siamese army was surprised that the Burmese had launched a two-pronged attack as they were only expecting the Burmese to only attack from the Three Pagoda Pass.
Capture of Phitsanulok Bayinnaung's army consisted of 60,000 men, 2,400 horses, 360 elephants, and another army from Lan Na These forces marched towards the capital city
Ayutthaya, but first faced forces at the city of
Phitsanulok. The governor of Phitsanulok,
Mahathammarachathirat initially resisted the Burmese attacks as the Burmese surrounded the city. However, seeing the hopelessness of the situation, dwindling supplies, and a smallpox outbreak,
Mahathammarachathirat surrendered on 20 February 1563.
Battle at Chai Nat After learning that Bayinnaung had brought the Burmese Royal Army down from the Hua Mueang Nuea with Siamese recruits, Chakkraphat ordered an army to be raised. Phraya Phichai Ronnarit and Phraya Wichit Narong were sent to set up camp at Nakhon Sawan while Prince Ramesuan's main army was to help relieve the siege of Phitsanulok. However, after learning that Phitsanulok had fallen, Prince Ramesuan decided to set up camp at Chai Nat to stop the Burmese advance. The Siamese forces used cannons on ships to blast at the Burmese army, causing many casualties. Bayinnaung later commanded the Burmese riverine fleet under Thado Dhamma Yaza I of Prome to attack the Siamese. As the Siamese were heavily outnumbered, they retreated back to Ayutthaya. ==Siege of Ayutthaya==