He came to office by assassinating his elder brother Thawun Gyi while Toungoo's overlord Pinya was facing a serious rebellion at Sagaing.
According to the regional chronicle Toungoo Yazawin, he was born in Pyu where his father had been placed under house arrest since 1256 by a rival governor from a Mon state to the south,[1] which most likely was Pegu (Bago).
[note 1] The fight between the rival governors took place during a brief interregnum following the death of King Uzana of Pagan (Bagan) c. May 1256.
At the capital Pagan (Bagan), a power struggle broke out between Crown Prince Thihathu and his half-brother Narathihapate.
[2][3] Since Thawun Letya remained under house arrest until his death in 1279,[4] he may have backed Thihathu.
Thihathu did send an army led by Crown Prince Uzana, which laid siege to Toungoo.
Thawun Nge successfully negotiated a deal to keep his office in exchange for his submission to Pinya.