Somdet Phra Borommarachathirat II (Thai: สมเด็จพระบรมราชาธิราชที่ 2) or Chao Sam Phraya (Thai: เจ้าสามพระยา) was the king of Ayutthaya. His reign saw its early expansions.
Chao Sam Phraya (lit. The Third Chao Phraya) was a son of Intha Racha who had finally taken the Ayutthayan throne for the Suphannaphum Dynasty. He had two elder brothers: Chao Ai Phraya (lit. The First Chao Phraya) and Chao Yi Phraya (lit. The Second Chao Phraya), and was appointed by his father to govern Chainat.
In 1424, Intha Racha died. His two brothers marched from their respective cities to Ayutthaya for the throne. They fought on elephants in single combat and both died, leaving the throne to Chao Sam Phraya. Chao Sam Phraya led Siamese forces to subjugate Cambodia plundering Angkor Thom. This assault eventually caused the Khmers to abandon Angkor and to relocate their capital further south-east.:29
The conquest, however, brought in a large influx of Khmer culture and traditions into the Siamese court. For example, the high reverence of Thai kingship as a deity, known as Devaraja, came from Cambodia.
Chao Sam Phraya also sought northward expansion. He married a daughter of the vassal Prince of Sukhothai, Maha Tammaraja IV, and had a son, who will grow up to be Prince Ramesuan.:31 When the last king of Sukhothai died in 1446, his grandson inherited the kingdom, further strengthening Ayutthaya control over Sukhothai.
In 1442, Chao Sam Phraya led his armies to conquer Lanna. He managed to sack the capital Chiang Mai but was unable to conquer the kingdom.
Chao Sam Phraya died in 1448 and was succeeded by his son Prince Ramesuan as Trilokanat.