By 1759, Alaungpaya had reunified all of Burma plus Manipur and Lan Na. However, his hold on Lan Na and Tenasserim coast was still nominal. The Siamese who originally were concerned about the rising power of Restored Hanthawaddy now actively supported the ethnic Mon rebels operating in the upper Tenasserim coast. In December 1759, Alaungpaya and his 40,000-strong armies invaded the Tenasserim coast. They crossed over the Tenasserim Hills, and finally reached Ayutthaya on 11 April 1760. But only five days into the siege, the Burmese king suddenly fell ill and the Burmese withdrew. The king died three weeks later, ending the war. The 1760 war was inconclusive. Although Burma had regained the upper Tenasserim coast to Tavoy, they still had to deal with Siamese supported rebellions in Lan Na  and at Tavoy . The warfare resumed in August 1765, when two Burmese armies invaded again in a pincer movement on the Siamese capital. The Burmese armies took Ayutthaya in April 1767 after a 14-months' siege. The Burmese armies sacked the city and committed atrocities that mar the Burmese-Thai relations to the present. The Burmese were forced to withdraw a few months later due to the Chinese invasions of their homeland. Burma however had annexed the lower Tenasserim coast.

What happened first: Burma was reunified or Burmese armies took Ayutthaya?
reunified all of Burma