The casus belli have been stated as a Burmese attempt to expand their territory eastwards after a political crisis in Ayutthaya as well as an attempt to stop Siamese incursions into the upper Tenasserim coast. The war began when a Burmese invasion force led by King Tabinshwehti and his deputy Bayinnaung invaded Siam through the Three Pagodas Pass. The Burmese forces penetrated up to the capital city of Ayutthaya but could not take the heavily fortified city. One month into the siege, Siamese counterattacks broke the siege, and drove back the invasion force. But the Burmese negotiated a safe retreat in exchange for the return of two important Siamese nobles (the heir apparent Prince Ramesuan, and Prince Thammaracha of Phitsanulok) whom they had captured.
The successful defense preserved Siamese independence for 15 years. Still, the war was not decisive. The next Burmese invasion in 1563 would force a Siamese surrender in 1564, and make Ayutthaya a vassal state of Burma for the first time. A Siamese revolt in 1568 would provoke a subsequent invasion, which resulted in the first-ever military capture of the capital in 1569, cementing Burmese rule for 15 years.
 Rise of TabinshwehtiBurmese city of Taungoo rebelled against his overlord at Ava, and crowned himself King Minkyinyo, establishing the Taungoo Dynasty and a new kingdom. His rebellion resulted in turmoil and conflict between rival factions within the Kingdom of Ava, which had been the dominant state in central Burma since 1364. The Saopha lords of the Shan States in the north saw an opportunity to invade, capturing Ava and annexing its territories in 1527. Dissatisfied with Shan rule, many of the Burmese nobility and commoners migrated with their families to Taungoo, thus turning it into a yet more powerful state.
In 1531, Minkyinyo died and his son Tabinshwehti succeeded him as King of Taungoo. In 1535, Tabinshwehti attacked the prosperous but declining southern Mon Kingdom of Hanthawaddy (commonly referred to as Pegu). By 1538, the majority of Mon territories, including the city of Pegu, were under Tabinshwehti's control. In 1539, he made Pegu his capital, and thenceforward was styled King of Pegu (or Hanthawaddy). The great Burmese towns of Prome and Martaban soon fell to his forces.
During these wars, Tabinshwehti's most able commander was a childhood friend, a son of his wet nurse. After marrying the King's sister Princess Thakhingyi, the general was given the name of Bayinnaung Kyawhtin Nawrahta (ဘုရင့်နောင်ကျော်ထင်နော်ရထာ) or Bayinnaung, which means 'Royal Brother-in-law'. Later, he was also made heir apparent and Crown Prince. Significantly for the Siamese, for the first time Burma and Siam shared a common border.
 Battle at the borderIn 1538, Tabinshwehti captured the Mon town of Gyaing (in Burmese—Chiang Kran in Thai), a Siamese tributary state on the border between Siam and Burma. Ayutthaya's King Chairacha (in full, Chairachathirat) mobilized his forces and moved towards Chiang Kran in November. Ayutthaya at that time had about 130 Portuguese mercenary-traders, and Chairacha retained 120 of these men to aid his campaign.
After the successful recapture of the town, the Siamese king, clearly impressed, praised the Portuguese for his victory. He rewarded them with a land grant, south of the city of Ayutthaya and east of Khlong Takhian canal at Ban Din, to erect St. Dominic's Church, the first permanent place of Christian worship in the kingdom. They also received permission to built their homes and factories near the new church. Prince Damrong Rajanubhab in his history, Our Wars With The Burmese (1917), called this battle "the first in which the Siamese and Burmese came to cross swords during the time when Ayutthaya was the capital of Siam".
 Crisis in AyutthayaChairacha of Ayutthaya was a scion of the Suphannaphum Dynasty, which took control of Siam from the Uthong Dynasty in 1409. He came to the throne in 1533 after usurping the crown of his five-year-old nephew, Phra Ratsadathirat, who had reigned for only four months. The boy's father was King Borommarachathirat IV, Chairacha's half-brother. The child-king was subsequently executed by his uncle. King Chairacha died in 1546 after reigning for thirteen years, leaving the throne to his eleven-year-old-son, Prince Kaeofa, who was crowned King Yot Fa.
As the new king had not come of age, the role of regent was assumed by his mother, Chairacha's chief consort (sometimes referred to as queen) Si Sudachan (ศรีสุดาจันทร์, also spelled Sri Sudachan), who was a descendant of the Uthong royal house. Chairacha's half-brother and Uparaja, Prince Thianracha, was another contender for the regency. To avoid court intrigues and conflict with Si Sudachan, Prince Thianracha retreated to a monastery as a monk. It was said that even before the previous King's death, Si Sudachan was having an adulterous relationship with a paramour styled Khun Chinnarat, who was keeper of the Royal chapel or cloister (หอพระเทพบิดร, Ho Phra Thep Bidon) within the Royal Palace of Ayutthaya. Fernão Mendes Pinto, a contemporary Portuguese explorer, recorded a rumour alleging that Si Sudachan had poisoned her husband in order to take control of the throne, and perhaps to restore the fallen House of Uthong to power. In support of these allegations, she had many prominent officials executed, including the aged and high-ranking Phraya Maha Sena, and replaced them with her favourites. It was also recorded that she was heavily pregnant and soon gave birth to a daughter; unable to conceal this secret, in 1548 she mounted a coup, removed her son and put her paramour on the throne. He was crowned on 11 November 1548 as King (or Khun) Worawongsathirat. It was said that the young King Yot Fa was either executed or poisoned by his mother.
Worawongsathirat's reign was short. Within 42 days several nobles and government officials of Ayutthaya plotted to remove him from the throne. The conspirators were led by Khun Phiren Thorathep, a descendant on his father's side to the kings of Sukhothai and a relation on his mother's side to King Chairacha. The usurper was lured from the safety of the palace into the jungle with a promise of capturing a large elephant. As the usurper king, Si Sudachan and their infant daughter proceeded by royal barge, Khun Phiren Thorathep and his conspirators sprang an ambush, killing all three. Prince Thianracha was immediately invited to leave the Sangha and assume the throne as King Maha Chakkraphat. One of his first acts was to appoint Khun Phiren Thorathep as King of Sukhothai (but as a vassal to himself) with a capital at the great fortified town of Phitsanulok. The king then bestowed upon him the title Maha Thammaracha (a title used by the last four kings of Sukhothai), along with the hand of his daughter Princess Sawatdirat in marriage.
 Invasion During the ten years that had elapsed since the battle at Gyaing, Tabinshwehti had completed his conquest of the Mon territories. He enlarged his battle-hardened army greatly, with new feudal conscripts and many Portuguese mercenaries. He also had Mon territories directly adjacent to Siam at his disposal, as a base from which to mount an effective invasion to capture Ayutthaya. According to Burmese chronicles, at the end of 1548 a small Siamese force attacked Tavoy, however they were easily repelled. Tabinshwehti demanded reparations for this incursion, when the Siamese refused—the war between the Siam and the Burma resumed. Tabinshwehti soon took personal command and gathered his forces at Martaban.
The invasion force would have been equipped with the conventional weapons of the day: swords, bow and arrows and spears. The more elite members would also carry matchlocks or muskets. These early modern weapons having been introduced to the two kingdoms by the Portuguese sometime earlier. Also, Diogo Soares de Mello, a Portuguese commanding a force of five captains and 180 professional mercenaries, was in Tabinshwehti's service. On top of this, the king also had a corps of Portuguese guards, numbering 400, whose morions and arquebuses were inlaid with gold. For the king they provided personal protection as well as expertise on artillery.
In January 1549, Tabinshwehti with his army began their invasion of Siam. Tabinshwehti invaded through a southern route, from Martaban along the Ataran river, over high ground toward the Three Pagodas Pass, and onto Siamese territory. The army then marched along the Khwae Noi River to the town of Sai Yok, then overland towards the Khwae Yai River; from there the army travelled by boat toward the town of Kanchanaburi. Tabinshwehti travelled in great state with a massive retinue of elephants and servants. Many of these elephants carried jingals and bronze cannon; these were kept close to the king. Royal elephants were rafted across rivers, while the ordinary war elephants marched upstream to a ford. The Burmese king was accompanied by his crown prince Bayinnaung, Bayinnaung's thirteen-year-old son Nanda, and many richly attired lords. Hundreds of workmen marched ahead of the king's retinue, to pitch a richly decorated wooden camp, painted and gilded for the King's use, only to pack it up and pitch it at a new location every day.
 Death of the queen Upon hearing of the Burmese invasion, Maha Chakkraphat mobilized his kingdom, then gathered his forces at Suphanburi, a town just west of Ayutthaya. When Tabinshwehti and his army arrived at the walled town of Kanchanaburi, they found it completely deserted. The King of Burma then continued his march eastward, capturing the villages of Ban Thuan, Kaphan Tru and Chorakhe Sam Phan. Tabinshwehti divided his army into three columns, the first commanded by Bayinnaung, the second by the Viceroy of Prome and the third by Yong, the Governor of Bassein. The Burmese continued their advance and captured the ancient town of Uthong as well as the villages of Don Rakhang and Nong Sarai and closing in on Suphanburi. When the Burmese attacked the town, Siamese defenders could not withstand the onslaught and retreated towards Ayutthaya. Tabinshwehti ordered his army southeast along two canals, and crossed the Chao Phraya river near Phong Phaeng. From here he encamped his army directly north of the Siamese capital of Ayutthaya on a field called the Lumpli plain.
On 3 February 1549, Maha Chakkraphat decided to leave the capital with his forces, to engage Tabinshwehti and test the Burmese strength. On this occasion, he mounted his chief war elephant. Accompanying him were his Chief Queen, Sri Suriyothai, and one of their young daughters, Princess Boromdhilok, the two riding together on a smaller war elephant. Both royal ladies were dressed in male military attire (helmet and armour), with the queen wearing the uniform of an Uparaja. Also accompanying their father on elephant mounts were two sons, the Uparaja and heir apparent, Prince Ramesuan, and his brother Prince Mahin.
The Siamese army under Maha Chakkraphat soon met the advance column commanded by the Viceroy of Prome, and the two armies engaged in battle. The commanders of the two forces engaged in single elephant-combat, as was the custom of the time. But Maha Chakkraphat's elephant panicked and gave flight, charging away from the enemy; the Viceroy swiftly give chase. Fearing for the life her husband, Queen Sri Suriyothai charged ahead to put her elephant between the King and the Viceroy, thereby blocking his pursuit. The Viceroy then engaged the Queen in single combat, fatally cleaving her from shoulder to heart with his spear, also wounding her daughter—both mother and child met their deaths on the back of the same elephant. It was said that the Viceroy did not know he was fighting a woman until his blow struck—as she fell dying her helmet came off, exposing her long hair.
Prince Ramesuan and Prince Mahin then urged their elephants forward to fight the Viceroy, drove him and his remaining forces from the field, then carried the bodies of their mother and sister back to Ayutthaya. The Siamese king meanwhile rallied his army, and retreated in good order back towards the capital.
 Attack on AyutthayaKing Tabinshwehti readied his army for a siege of the Siamese capital at the beginning of March. Tabinshwehti made his camp north of the city, with his headquarters at Klum Dong, and had his commanders encamp in strategic places surrounding the city walls, Bayinnaung at Phaniat, the Viceroy at Ban Mai Makham, and the Governor of Bassein at the plain of Prachet. The Burmese would not, however, take the Siamese capital so easily.
Ayutthaya sat on an island surrounded by three rivers—the Lopburi River to the north, the Chao Phraya River to the west and south, and the Pa Sak River to the east, forming a formidable natural moat. The Chao Phraya basin where Ayutthaya is situated was low and prone to flooding—especially intense during the rainy season when torrential waters flowed in great quantity from the north along the Lopburi River. This flood would begin approximately in July and end somewhere between October and November, giving Tabinshwehti only five months to capture Ayutthaya—otherwise his camp grounds and supply routes would be flooded. There was also the possibility that the flood could trap his forces. The low, swampy area around the city was laced with numerous canals thronging with gun boats armed with cannon to repulse any attempt at an attack on the city. Also, the Burmese had only small cannon that they had brought with them, while the Siamese had large cannon mounted along the city walls. The Burmese had the city surrounded, but without the ability to cross the rivers or breach the city walls with cannonfire, were left to camp around it instead, while the interconnected waterways to the north and south made it fairly easy to resupply the defenders in the city. Fifty Portuguese mercenaries, who had elected Galeote Pereira as their captain, defended the weakest part of the city wall for Maha Chakkraphat. Unable to take the city conventionally, Tabinshwehti offered bribes to these defenders. The Portuguese reacted with derision, and refused. When a Siamese commander heard of this, he swung open the gates of the city and dared the Burmese King to bring the money—a dare that was ignored.
Maha Chakkraphat, being unable to repel the Burmese, sent a message to his son-in-law Maha Thammaracha at Phitsanulok, ordering his vassal to come to his aid by bringing an army southwards towards Ayutthaya and if possible to engage the enemy in battle. Thammaracha quickly mobilized his forces and with the help of the Governor of Sawankhalok, marched southward with a large army to attack the Burmese rear. Upon hearing of this and on the advice of Bayinnaung; Tabinshwehti decided to withdraw, abandoning the mission altogether. His decision was compounded by news from Burma that the Mons, who had never been entirely subjugated by the Taungoo dynasty, rebelled in the absence of the king. Other factors included the scarcity of supplies and sickness in his army, which was not prepared for a long siege. Only one month into the siege (around April), Tabinshwehti withdrew his forces towards the border.
 Retreatretreat back through the Three Pagodas Pass, along the same route he has taken for the invasion. This proved difficult as food and supplies in the land were scarce, so he went north by the way of the Mae Lamao pass (in modern day Mae Sot, Tak). As they withdrew, the Burmese tried to plunder the ancient and wealthy town of Kamphaeng Phet, but the town was too well fortified. With the help of more Portuguese mercenaries, the Governor repelled the Burmese with flaming projectiles that forced the Burmese to cease using their cannons and protect them with coverings of damp hides.
Maha Chakkraphat saw the Burmese army's retreat as an opportunity take advantage of their weakness, so he ordered Princes Ramesuan and Thammaracha to follow and harass the enemy out of Siamese territory. For three days, the Siamese chased Tabinshwehti and his forces, inflicting great losses upon them. Once the forces of Ramesuan and Thammaracha closed in, Tabinshwehti elected to stand ground and ambush them near Kamphaeng Phet, dividing his forces on both sides of the road. The Siamese in their eagerness fell into the trap. The Burmese captured both Prince Ramesuan and Maha Thammaracha as prisoners of war.
The capture of his heir and his son-in-law forced Maha Chakkraphat to negotiate with Tabinshwehti. The Siamese at once sent emissaries bearing gifts, offering a peaceful retreat in return for the two princes. In exchange Maha Chakkraphat was forced to hand over to Tabinshwehti two prized male war elephants called Sri Mongkol (ศรีมงคล) and Mongkol Thawip (มงคลทวีป). Once the elephants were handed over, the Burmese army retreated in peace. In addition to the two princes, Tabinshwehti also released many other prisoners he had captured during the campaign. All in all, the campaign from beginning to end lasted five months.
 Legacy Burmese chronicles recorded that Tabinshwehti, after his return to Pegu, took to drink and abandoned his royal duties, leaving the running of the government to Bayinnaung who acted as Regent. In 1550 when Bayinnaung had left the capital to fight a Mon rebellion in the south, Tabinshwehti was murdered and the throne of Pegu usurped, leading to a full blown rebellion that took Bayinnaung five years to quell, ending with the capture of Ava in 1555.
The body of Queen Sri Suriyothai was placed at Suan Luang, the Royal Garden. Maha Chakkraphat ordered a grand cremation, and built a temple with a large stupa to house her remains. The temple, which still exists, is known as Wat Suan Luang Sop Sawan (วัดสวนหลวงสบสวรรค์) and the stupa is called Chedi Phra Sri Suriyothai (เจดีย์พระศรีสุริโยทัย). The temple and the stupa had been restored and rebuilt several times.
historicity of her story and her existence has been the subject of debate. This is based on the fact that the queen is not mentioned in either the recorded or popular history of Myanmar. All the facts pertaining to her life were taken from fragments of the Siamese royal chronicle the Annals of Ayutthaya and a account by Domingos Seixas (a Portuguese explorer).
The war led to the strengthening of Ayutthaya's defences, such as stronger walls and forts. A census of all able-bodied men was taken, as well as a massive hunt for wild elephants for use in future wars. The size of the navy was also increased.
The Siamese success at repelling the Burmese would not be repeated. This first ever invasion gave the Burmese an important experience on fighting with Siamese. The next invasion would be conducted by Bayinnaung, a man accustomed to fighting against Siamese soldiers and familiar with marching through Siamese terrain. The unrest in Burma delayed that next invasion for fifteen years, until the War of 1563 or the War of the White Elephants.
 MediaThe war beginning with the death of Chairacha was dramatized in the 2001 Thai historical drama The Legend of Suriyothai, directed by Mom Chao Chatrichalerm Yukol. The film portrays the events leading up to the war and the battles including the death of Queen Sri Suriyothai. The film cost an estimated 350 million baht, and is the highest budget Thai film to date. The film was released in the United States in 2003.
The succession crisis Ayutthaya is portrayed in the 2005 English language Thai film The King Maker. However, the film ends prior to the Bur