Thado Dhamma Yaza III of Prome

Hnaung's first major assignment was to be king of Lan Xang. In September/October 1588, Pegu's vassal king Maha Ouparat of Lan Xang died.[6] Nanda appointed Hnaung the next king of Lan Xang although he had reservations about sending his teenage son to the remote rebellion-prone kingdom. But in November/December,[7] Thado Dhamma Yaza II, Viceroy of Prome, died, and Nanda changed his mind and appointed Hnaung at Prome, a strategic vassal state (present-day western Bago Region and Magwe Region). On 26 February [O.S. 16 February] 1589,[note 2] Hnaung became viceroy with the title of Thado Dhamma Yaza III. His official style was King of Prome.[8]

At Prome, one of his main responsibilities was to contribute manpower to his father's increasingly stretched military. At least in the beginning, the young viceroy apparently did not have much control over his own vassal rulers, a group that included Minye Uzana of Salin and Min Shwe Myat of Taingda, sons of the previous viceroy Thado Dhamma Yaza II. In 1590, he could contribute just a regiment to a 24-regiment (20,000-man) invasion force of Siam.

But he still helped out his father in some other way. He agreed to lead an expedition to suppress a rebellion in the Shan state of Mogaung (modern Kachin State). On 1 November [O.S. 22 October] 1590, he led a 10,000-man army, largely made up of conscripts from Upper Burma, and left for the front. Under his command was an eleven-year old Natshinnaung, his second cousin and the heir-presumptive of Toungoo (Taungoo).[9] (The two grandsons of Bayinnaung would later become adversaries less than five years later.) The campaign was a success. His forces retook the fortified town in early 1591. Hnaung arrived at Pegu triumphantly in April 1591[10] but the capital was in no mood to celebrate. The fourth invasion of Siam had gone terribly wrong. The Burmese forces led by Crown Prince Mingyi Swa had been thoroughly routed in northern Siam by the Siamese army led by King Naresuan.[11]

Hnaung was asked to join the debilitating war effort. By then, the tide was turning in favor of the former vassal state. In the dry season of 1591–92, Siam invaded Burma for the first time. Naresuan and his army advanced as far as Martaban (Mottama) before they were driven back. The Tenasserim coast now fell under Siamese control. The Pegu court responded by planning yet another invasion of Siam. Mingyi Swa would again lead the invasion with Hnaung and Natshinnaung as deputies. On 14 November [O.S. 4 November] 1592, three armies with a combined force of 24,000 men, 2000 horses, 150 elephants, left Pegu. The invasion came to a sudden halt on 8 January [O.S. 29 December 1592] 1593 when Mingyi Swa fell in action near Suphan Buri. Hnaung took over as the overall commander-in-chief, and decided to retreat.[12]

The failed invasion further diminished Nanda's stature with his vassals. Worse yet, the High King's ability to project power was now severely hampered: Lower Burma, the only region over which the high king had direct control, was heavily depopulated.[13] Tired of constant war, able men had fled military service to become monks, indentured servants, private retainers or refugees in the nearby kingdoms.[14] Hnaung himself became disillusioned when the new Crown Prince Minye Kyawswa frantically set out to raise more men—again mainly from Lower Burma by branding men to facilitate identification, executing deserters, and forcing monks into the army.[14] By late 1594, he openly clashed with his brother over the latter's aggressive tactics. Nanda was forced intervene, officially releasing the conscripted men and seized property.[15]

But the damage was already done. The viceroy of Prome appeared to have decided to break away as early as late 1594. When the Siamese army invaded the Upper Tenasserim coast in October/November 1594,[16] Nanda asked his vassals to send troops. Hnaung now behaved like most other vassals, promising to send help but never did.[17] He did nothing even Siamese forces laid siege to Pegu in December. He gave the excuse that his army would coordinate with the armies from Toungoo (Taungoo) and Lan Na, and they would jointly break the siege.[17]

By then, Hnaung had already decided to revolt. He plotted to gain control the Myelat (မြေလတ်, "Middle Country") region by seizing Toungoo. Over three months into the siege, in March 1595, combined Toungoo and Lan Na armies finally marched to Pegu from Toungoo. After getting the confirmation that Viceroy Minye Thihathu II of Toungoo had left with the army, Hnaung too began his march—not to Pegu but to Toungoo. As the combined Toungoo–Lan Na armies approached the Ayutthaya armies outside Pegu, Prome forces showed up at the gates of Toungoo c. 26 March  [O.S. 16 March] 1595.[note 3] But Toungoo was heavily fortified, and equipped with artillery and musket corps. Its defenses were led by Natshinnaung, the heir-apparent of Toungoo. Hnaung laid siege to the city, counting on Minye Thihathu to be occupied or defeated by the Siamese army. But it was not to be. The combined armies broke the siege on 8 April  [O.S. 29 March] 1595.[note 4] Hnaung decided to retreat as Minye Thihathu rushed back to Toungoo. Upon his arrival at Prome, he officially declared independence from his father.[17]