The crisis escalated in 1385 when Viceroy Laukpya of the Province of Myaungmya persuaded King Swa Saw Ke of Ava to put him on the Hanthawaddy throne.

When his bickering sons came back empty-handed after a badly coordinated campaign five months later, Swa himself invaded with much larger land and riverine forces.

When Ava entered into a succession crisis in 1400, Razadarit felt powerful enough that he invaded his larger and more populous northern neighbor, starting the Ava–Hanthawaddy War (1401–1403).

The origins of the war trace back to the two kingdoms' lukewarm relationship since the early 1370s as well as Ava's long-running problems with its northern neighbor, Shan-speaking Maw (Mong Mao).

[16][17] When Ava's founder Thado Minbya died in 1367, his successor Swa Saw Ke inherited a largely embryonic polity that controlled few peripheral regions.

[18][19] On the southern coast, the polity based out of Pegu (Bago) came into being as a remnant of the Martaban Kingdom in 1369/70 when King Binnya U was forced to retreat to the hitherto provincial capital by his cousins Byattaba and Laukpya, who had waged a rebellion since 1364.

Though U regained Donwun from Byattaba in 1371,[25] the king was soon forced to make peace when his cousins sought an intervention by the kingdom of Lan Na (in present-day northern Thailand).

[35] Pegu's succession crisis began in May 1383 when Prince Binnya Nwe raised a rebellion against his ailing father (and his aunt and adoptive mother Princess Maha Dewi).

I, Laukpya, Lord of Myaungmya, bowed at thy feet, O king of the Golden Palace at Ava, and pray that thou march against Razadarit before he is firm upon his throne.

[40][42] Swa, who wanted to restore the erstwhile Pagan Empire, accepted Laukpya's proposal, touching off what would become "the forty years' war between Ava and Pegu.

The first army led by the crown prince (9 regiments, 7,000 infantry, 500 cavalry, 20 war elephants) was to invade the Sittaung river valley from Toungoo.

On the northeastern front along the Sittaung river, Hanthawaddy's main defenses began only at Fort Pankyaw (modern Waw),[note 8] about 40 km (25 mi) northeast of Pegu.

On the northwestern and western fronts, a series of smaller forts at Hlaing (modern Taikkyi), Hmawbi, and Dagon defended the route towards Pegu.

The riverine force (10 regiments, 17,000 troops, 100 transport boats/ships, 120 war boats) commanded by King Swa himself would attack the southern ports via the delta.

After weighing the competing demands, Swa concluded that the developing situation must be taken care of first, and ordered an expedition to the north in the following 1387–1388 dry season.

Realizing that they needed to control as many resources as possible to fend off Ava in the long run, Razadarit and his staff set out to reunify the provinces.

In the subsequent battle, the Hanthawaddy army was nearly defeated but the two rearguard regiments led by Than-Byat and Dein managed to drive out Byattaba and his senior staff from the battlefield.

[note 16] The king ordered repeated frontal assaults but the city's defenses repelled the charges each time, inflicting heavy casualties on the Hanthawaddy troops.

Razadarit finally called off the attacks after one of his frontline commanders, Yaza Manu, was felled atop his war elephant by a gun shot.

Razadarit acknowledged that he had severely misjudged the relative strength of Bassein, and that his degraded forces were in no shape to take on Myaungmya's even stronger defenses.

His senior staff, led by Byat Za and Dein Mani-Yut, argued that the only way they might have a chance at victory was if they could entice the enemy to come out of their fortified cities.

The king agreed, and ordered his troops to withdraw into the main corridor between Bassein and Myaungmya, which were about 50 km (31 mi) apart, with the hope that the enemy forces would come after them.

Bassein's naval flotilla pursued Lagun Ein's squadron until the pursuers ran aground at Pan Hlaing, where they were ambushed from both sides by more Hanthwaddy war boats in the wings.

Dein, with his 1,000-man regiment stayed behind, and sent a letter to the fort's commander Saw E Binnya, another of Laukpya's sons, that he had defected and wanted to join the Myaungmya side.

As the opposing forces clashed right outside the city's gates, Razadarit and Pyit-Nwe eventually became entangled, fighting an impromptu duel on their respective war elephants.

In the subsequent battle, both sustained wounds but Razadarit emerged victorious when Pyit-Nwe was thrown off his elephant, and captured by the Hanthawaddy infantry nearby.

[65] Razadarit dispatched Dein and Byat Za to intercept Lauk Shein, forcing him to change course to Sandoway (Thandwe), 300 km (190 mi) northwest of Bassein, in Arakan.

On the Irrawaddy front, after Hanthawaddy forces evacuated Gu-Htut, Swa deployed a garrison led by Zeya Gamani and Yaza Nawrahta.

In 1392, Maw forces led by the sawbwa of Mohnyin invaded, routed Ava's northernmost garrison at Myedu, and advanced as far south as 30 km (19 mi), northwest of Sagaing.

His administration consisted mainly of ministers from his father's court led by Dein Mani-Yut, Byat Za, Zeik-Bye and Maha Thamun.