Constantly plagued by poor health, U increasingly relied on his sister Maha Dewi to govern.
He formally handed her all his powers in 1383 while facing an open rebellion by his eldest son Binnya Nwe, who succeeded him as King Razadarit.
One enduring legacy of his reign was Pegu's (Bago's) emergence as the new power center in Lower Burma.
Born c. late 1323,[note 1] Binnya U (Mon: ဗညာဥူ[1]) was the only son of Queen Sanda Min Hla and King Saw Zein (r. 1323–1330) of Martaban.
[8] He actually built a Buddhist pagoda, housing a holy relic he had received from Ceylon,[8][6] at the hilltop where he atop his elephant began the counterattack.
[13] When Theingaba formally revolted against Pinya in 1358, he stayed on good terms with the rebel governor but did not want Toungoo to become too strong either.
[14] By the early 1360s, both Pinya and Sagaing kingdoms had been under repeated raids by the northern Shan state of Maw.
According to the chronicle Razadarit Ayedawbon, the main cause was the king's penchant for months-long elephant hunting trips away from the capital.
[16] Four months into the trip, c. February 1364,[note 3] he received news that a coup led by princes Byattaba and his brother Laukpya had taken place at the capital.
His hold over the Pegu province was helped by the effective rule of his elder sister, Maha Dewi, whom he appointed governor of Dagon in 1364.
When he heard that Byattaba was seeking Lan Na's help, U tried to repair relations with the Tai kingdom by sending his eldest daughter Tala Mi Thiri to Chiang Mai in a marriage of state.
(Queen Thiri would have a terrible time at Chiang Mai, and constantly urged her father to bring her back.
Taking advantage of the ensuing commotion, Byattba sent 700 warriors disguised as mourners, who managed to get inside Donwun, and seize U's palace.
The king barely escaped to the nearby woods, and fled to Pegu,[21] about 120 km northwest of Donwun, across the Bay of Martaban.
[23] Indeed, all subsequent monarchs of the Wareru dynasty would remain at the more strategic Pegu even after Martaban returned to the fold.
In 1370–71, he quickly signed a peace treaty with King Swa Saw Ke of the emerging Ava Kingdom in the north.
He successfully fended off Laukpya's attempts to seize southern Pegu province, and his forces even retook Donwun, restoring the pre-1369 borders.
As the Razadarit Ayedawbon recounts, U realized the absurdity of the overlord paying tribute to the vassals but he ultimately decided to swallow his pride, and agreed to the terms.
[30] On the other hand, Byattaba and Laukpya realized that they needed Lan Na's support in order to neutralize U's stronger position at Pegu.
In 1375, Ava, coming off a decisive victory over the northern Shan state of Mohnyin,[31][32] sought to impose tighter control over Toungoo.
Pegu sent a sizable army that included cavalry and elephant units led Commander Ma Sein.
[33][34] According to a contemporary inscription, by the time the dust had settled, Ava had sent three expeditions against Toungoo, and caused widespread starvation in the region.
He gradually withdrew from administration, and handed increasing responsibilities to his sister Maha Dewi, culminating in her appointment as regent in 1383, and his eldest son Binnya Nwe's rebellion.
When his health, in poor state at least since 1364,[note 5] deteriorated further in the 1370s, he began asking her to take on administrative duties.
Her enemies tried to undermine her by making public her alleged affair with a much younger married Smim Maru, who was also her nephew-in-law and U's son-in-law.
Prince Nwe, whose mother died at childbirth, had been raised by his aunt Maha Dewi since his birth.
Maha Dewi had to repeatedly plead with her brother to free Nwe, and allow the young couple to be married.
[43] It was only at the end of August that she decided to use force,[44] after learning that Nwe had sent missions to enlist help from Martaban and Myaungmya.
In October, he officially handed power to his sister, giving her the right to raise the white umbrella, a symbol of Burmese sovereigns.
The new king, known by the title of Razadarit, decided not to punish his adoptive mother, and reappointed her to her old post at Dagon but strictly in a ceremonial role.