The uneasy status quo did not last. In 1385, Laukpya on his own sought Ava's military assistance to oust Razadarit, offering to hold Pegu as a vassal state. The proposal resonated with the Ava court. By then, Ava had emerged as the main power in the Irrawaddy valley, with its king
Swa Saw Ke intent on restoring the erstwhile
Pagan Empire. Swa and his chief minister
Min Yaza foresaw little difficulty in taking over Pegu. Swa accepted Laukpya's proposal. What followed was the
Forty Years' War between Ava and Pegu. Swa sent down two armies (with a combined strength of 13,000 men, 1000 cavalry, 40 elephants), led by Swa's two young sons,
Tarabya and
Min Swe. Laukpya sent in a flotilla from the delta. At Pegu, Razadarit scrambled to raise manpower. In desperation, he sent a last-minute mission to Donwun in search of levies but Sam Lek ignored the request. With limited troops available, the Pegu command had no choice but defend from inside its key fortified towns. Ava forces went on to occupy much of the Pegu province but they could not break through the key forts. Their only chance came five months into the invasion in May 1386 when Razadarit came out of Pegu to attack the Ava-occupied Fort
Pankyaw. Ava forces quickly converged and nearly cornered Razadarit but Prince Min Swe's failure to follow orders allowed Razadarit to get back inside Pegu. Five days later, with the rainy season only weeks away, Ava forces withdrew. At Ava, Swa and his court were grappling with twin problems: in addition to Pegu, their appointed ruler of Arakan had also been driven out of
Launggyet. Swa was initially receptive to Pegu's offer. But after deliberating with his advisers for a month, he ultimately concluded that Pegu was still winnable outright, and an easier task than Arakan. Swa rejected Razadarit's proposal, and planned to lead the next invasion by himself. Razadarit had assumed the worst, and was prepared. His plan was again to outlast the invaders from inside the walls. In late 1386, 29,000 Ava troops led by Swa and Tarabya invaded by the
Irrawaddy and by land. Laukpya again joined the invasion with his flotilla. Yet, the invaders again could not break through the Hanthawaddy's defensive line at
Dagon,
Hlaing,
Dala and
Hmawbi, and had to retreat at the onset of the rainy season of 1387. It was the opening Razadarit and the court were looking for. Realizing that they needed to control as many resources as possible to fend off Ava in the long run, they set out to reunify the provinces. Over the next three years, while Ava was at war with Mohnyin, Razadarit successfully reunified his kingdom.
Donwun and Martaban Razadarit's initial goal was modest, targeting just the northern Martaban province. For their part, the motley crew rebels there remained unimpressed by the Pegu army, untested outside the walls. The rebels actually put up a fight, forcing Pegu forces to storm town after town. But the Pegu army proved its worth; able commanders such as
Than-Lan, Dein and
Lagun Ein emerged. Over the next few months, larger forces of Pegu eventually captured the entire northern province, including Donwun, a strategic town north of
Martaban (Mottama), the former capital of the kingdom. Buoyed by the initial success, Razadarit sent a 7000-strong army to probe Martaban's formidable defenses. His father could not retake Martaban in 1364–1371 but the outcome this time was to be different. Viceroy Byattaba of Martaban decided to come out of the city to ambush the Pegu army en route. The Pegu army was nearly defeated when the two rearguard regiments led by Than-Lan and Dein managed to drive out Byattaba from the battlefield. The army then raced to Martaban where the defenders quickly surrendered. The rest of the southern vassals also submitted. It was 1388 (or early 1389). After the improbable victory, an overjoyed Razadarit appointed Than-Lan governor of Donwun with the princely title of Smin Byat Za. In late 1389, Razadarit invaded the delta. Ava was still at war in the north, and could not send any help. Laukpya had to defend from inside the two heavily fortified port cities of
Myaungmya and
Bassein (Pathein). The strategy worked for awhile, holding off Razadarit's numerically superior forces for months. But as in Martaban, Laukpya's forces made what proved to be a crucial mistake of venturing out of Myaungmya. Laukpya's son and best commander
Ma Pyit-Nwe was captured. A panicked Laukpya tried to flee the city but was also captured. Two days after Myaungmya's fall, Bassein also fell. Razadarit's forces went on to occupy the entire delta, including Ava's territory,
Gu-Htut (modern Myan-Aung), in the northern delta, in 1390. His forces chased
Lauk Shein, governor of Bassein, all the way to
Sandoway (Thandwe) in Arakan, and laid siege to the city until Sandoway's ruler gave up Lauk Shein and his men. It was a total rout. Only Laukpya's son
Bya Kun and son-in-law
Bya Kyin escaped to Ava. Lauk Shein was executed at Razadarit's command. To govern the province, Razadarit appointed Byat Za, who led the conquest of the delta, governor of Myaungmya, and
Thilawa, a minister at the Pegu court, governor of Bassein. In recognition of the momentous occasion, he held a second, much grander coronation ceremony in Pegu. He again had Piya Yaza Dewi by his side as the chief queen. This led to another round of bitter arguments between him and his first wife. He now took away Daw's family heirlooms—a dozen rings given by their father King Binnya U—and gave them to Piya Yaza Dewi. It was not all. Razadarit also ordered their only child
Baw Law Kyan Daw executed. As recounted by the chronicle
Razadarit Ayedawbon, he reasoned that as he himself revolted against his father, his son would certainly avenge his mother when he grew up. According to legend, the child before his death proclaimed his innocence, and famously swore to be reborn as a prince of Ava and fight his father. Razadarit was greatly disturbed when he heard of the terrible oath. (In the superstitious world of Burmese politics, he was alarmed when the wife of Prince Min Swe of Ava gave birth to a son about a year later.)
Third Ava invasion (1390–1391) Despite the grandiose coronation, Razadarit was not yet out of danger. His victory in the delta had come just as Ava reached a truce with Mohnyin. (In a marriage of state, Princess
Shin Mi-Nauk, daughter of King Tho Ngan Bwa of Mohnyin, was wedded to Prince Min Swe, son of King Swa of Ava. The couple would have their first son, who according to legend, was Baw Law Kyan Daw reincarnated.) King Swa Saw Ke now turned his attention to the south; Pegu's occupation of Gu-Htut must be responded to. But unlike with the previous invasions, Ava forces faced far more numerous southern forces, who fiercely defended the border. The Hanthawaddy navy held off repeated charges by the larger Ava navy near Gu-Htut. Likewise, the Hanthawaddy army at Fort Pankyaw stopped the Ava army. Despite the military success, the Pegu court persuaded their king to return Gu-Htut to Ava in exchange for Ava's recognition. Faced with yet another embarrassing failure, Swa accepted the face-saving proposal. In early 1391, Razadarit and Swa agreed to a peace treaty that allowed Pegu to consolidate most of its gains. The peace almost broke down the following dry season. In late 1391, Ava posted an army in
Tharrawaddy (Thayawadi), its southernmost territory. In response, Pegu sent a sizable force to the border in a show of force. The fragile peace held. ==Height of power==