Khmer - Thai war of 1352 - 1444
In 1350, Ayutthayan King, Rama Thibodi I, born in 1312, began invading the Khmer Empire (Cambodia) to the east About 1352 Rama Thibodi put his son Prince Ramsuen, governor of Lop Buri, in charge of the invading forces. Prince Ramasuen sent part of his forces ahead - a strategic mistake; the Khmer Crown Prince was able to stop the invasion by defeating the 5,000 advance troops.
Upon hearing news of this Rama Thibodi sent Prince Boromoraja I, governor of Supan, to rescue the stalled expedition. He achieved victory over the Khmers and annexed the Khorat and Chantaburi districts. In 1369, Ayutthayans occupied the Khmer capital of Angkor, and in 1389; frequent Ayutthayan and Cham attacks threatened to destroy the hydraulic irrigation system that sustained the Khmer Empire, which was further weakened by intermittent Ayutthayan participation in Khmer royal politics and dynastic infighting, and loss of soldiers as prisoners of war.
Thai War of 1371 -1378 (King Boromoraja I)
King Boromoraja of Ayutthaya led a military expedition north to reestablish control of the Thai Kingdom of Sukhothai which was declining under the reign of King Thammaraja II and was in revolt. To discourage possible Chinese involvement in the conflict, King Boromoraja sent gifts to the Chinese Emperor in Nanking. In 1372, Boromorajas forces seized the towns of Muan Nakhon, Phangkha and Sengchangrao, where King Boromoraja had the chieftain Sai Keo executed and drove out the other Chieftain, Kham Heng with his army. King Boromoraja ceased hostilities for two years until returned to rebellious Sukhothai, taking control of the city of Pitsanulok, capturing Chieftain Khun Sam Keo (died in 1376), and enslaving many of the cities residents.
In 1376 King Boromoraja attacked Chakangrao, defended by Kham Heng and his forces which were not allied with the Thai Kingdom of Chiengmai to the north (Chiengmais king hoped to ensure his realms safety by maintaining Sukhothai as an independent buffer state between Chiengmai and Ayutthaya). The allied army attempted to ambush Boromorajas forces, but failed; its soldiers were slaughtered in a battle near Kamphaeng Phet. The Sukhothai chieftain Thao Phadong escaped but again, was defeated in battle and many Sukhothai officials were captured.
In 1378, King Boromoraja besieged Kamphaeng Phet once again, however, Thammaraja agreed to peace terms; his surrender and cession of western areas, including Kamphaeng Phet, to Ayutthaya. Thammaraja was allowed to rule Sukhothai under Ayutthayan suzerainty from a capital at Pitsanulok.
In 1430, Ayutthayan forces under King Boromoraja II besieged Angkor for seven months, finally capturing the city through the treachery of two Buddhist monks and Khmer officials, who later in 1444 after the death of Khmer king Dharmasoka, secretly joined the Ayutthayans. Angkor was sacked by the Ayutthayans, who were driven out in 1432 but returned later.
Thai war of 1387 - 1390
14 year old Sen Muang Ma born in 1373, ascended the throne of Chiengmai. His uncle, Prince Phrohm, began plotting to seize the crown and when he unsuccessful, called on King Boromoraja for help. Boromoraja sent an army, hoping to secretly gain control of Chiengmai. he opposing forces met at the Battle of Sen Sanuk, near the city of Chiengmai. Sen Muang Mas forces were victorious, and Boromoraja withdrew from the country.
Seng Maung Ma and his uncle resolved their differences; the latter joined the royal retinue and gave his nephew, as a token of friendship, a holy Buddha statue he had stolen from Kamphaeng Phets ruler, who protested the theft and the occupation of his city by Prince Phrohms forces. In response, Boromoraja led troops on a march to help Kampaeung Phet, which he captured; afterward King Boromoraja died en route home in 1388.
Two years later Sen Muang Ma led troops ostensibly to help the Kingdom of Sukhothai regain independence from Ayutthayan domination. But Sukhothais King Thammaraja II learning about a possible takeover of his kingdom by Chiengmai troops, confronted and severely defeated Sen Muang Mas forces. During the bloody battle Sen Muang Mas was forced to flee on the backs of two servants in order to avoid capture.
Thai War of 1411
Following the death of Chiengmais King Sen Muan Ma in 1411, a succession dispute arose between his sons, Prince Sam Fang Ken and his brother Prince Yi Kumkam. The latter prince requested help from King Intharaja I of Ayutthaya, who sent an army under King Thammaraja II of Sukhothai to place Prince Yi Kumkan on the throne of Chiengmai. To besiege and shoot into town of Phayao, King Thammarajas troops erected a 72 foot high earthen hill fort nearby and fought a battle in which both sides are allegedly used cannons. Phayao defenders, who melted down brass tiles to make cannon, repulsed the attackers and destroyed their fort. A short time later the Ayuttayans laid siege to the capitol city of Chiengmai, which fiercely resisted all attempts to take it.
Finally Sam Feng Ken suggested that the succession dispute be settled by a single contest between two champion warriors - one from Chiengmai and the other from Ayutthaya; Yi Kumkam agreed to renounce his claim to the throne if his warrior (the Ayutthayan) lost. The contest to be fought between both champion fighters was Muay Chao Cherd known today as Muay Thai or (Thai-Boxing), and was to be fought under the rule of first blood drawn. After several hours of combat in the hot sun, with both fighters tired, the Chiengmai champion was declared the winner when the Ayutthayan champion stepped on a sharp stone causing his big toe to bleed. The rules of the contest was acknowledged on first blood drawn. Consequently, the Ayutthayans withdrew from Chiengmai, where they carried off many captives to be slaves in Ayutthaya (indemnification for the cost of the war).