The subsequent years saw constant warfare as numerous states attempted to exploit the collapse of Khmer hegemony. As none of the parties in the region possessed a technological advantage, the outcome of battles was usually determined by the size of the armies. The use of war elephants continued, with some battles seeing personal combat between commanders on elephants. In 1592, Nanda Bayin king of Toungoo ordered his son Mingyi Swa to attack Ayutthaya Naresuan encamped his armies at Yuddhahatthi. The Burmese then arrived, leading to the Battle of Yuddhahatthi. Naresuan was able to killed Mingyi Swa while both were on elephants during the battle. After that Burmese army withdrew from Ayutthaya in 1599, Naresuan occupied the city of Pegu but Minye Thihathu Viceroy of Toungoo took Nanda Bayin and left for Toungoo. When Naresuan reached Pegu, what he found was only the city in ruins. He requested Toungoo to send Nanda Bayin back to him but Minye Thihathu refused. After each victorious campaign, Ayutthaya carried away a number of conquered people to its own territory, where they were assimilated and added to the labour force. To the south, Ayutthaya easily achieved domination over the outlying Malay states. To the north, however, the kingdom of Burma posed a potential military threat to the Siamese kingdom. Although frequently split and divided in the 16th century, during periods of unity Burma could, and did, defeat Ayutthaya in battle, such as in 1564 and 1569. What followed was another prolonged period of Burmese disunity.