The war began in 1547. The casus belli have been stated as an attempt by Toungoo Dynasty of Burma to expand its territory eastwards after a political crisis in Ayutthaya as well as an attempt to stop Ayutthaya's incursions into the upper Tenasserim coast. According to the Burmese chronicles, a Siamese force of 6000 had occupied Tavoy  in the Upper Tenasserim coast, which he considered his territory, by January 1547. As frontiers in the pre-modern period were less defined and often overlapped, the "occupation" may have been an attempt by Ayutthaya to reinforce the frontier town, which was claimed by Toungoo. At any rate, Tabinshwehti sent a sizable force of 12,000  led by Saw Lagun Ein, viceroy of Martaban, to take over Tavoy c. October/November 1547. A joint land-naval attack on Tavoy drove out the Siamese forces led by the Lord of Kanchanaburi to lower Tenasserim.
What happened first: war began or Tabinshwehti sent a sizable force?
war began