Back from Arakan, Tabinshwehti looked east to Siam, which had occupied what he considered his territory. His response to "Siamese incursions" would launch the centuries-long Burmese-Siamese Wars between Burma and Siam. Siamese sources deny that Siam began the hostilities; rather, it was Burma's attempt to expand its territory eastwards taking advantage of a political crisis in Ayutthaya that started the hostilities.  The Burmese king sent a sizeable force  led by Gen. Saw Lagun Ein of Martaban to drive out the Siamese forces from Ye and Tavoy in late 1547. Saw Lagun Ein's forces defeated Siamese forces led by the governor of Kanchanaburi, and retook down to Tavoy. Tabinshwehti was not satisfied, and planned an invasion of Siam itself. Next year, near the end of the rainy season on 14 October 1548, , 12,000 strong Toungoo forces led by Tabinshwehti and Bayinnaung invaded Siam via the Three Pagodas Pass. The Burmese forces overcame Siamese defences, and advanced to the capital city of Ayutthaya. But they could not take the heavily fortified city. One month into the siege, in January 1549, Siamese counterattacks broke the siege, and drove back the invasion force. On retreat, the Burmese tried to take Kamphaeng Phet, but it too was well defended by Portuguese mercenaries. Fortunately for the Burmese, they caught two important Siamese nobles  in some open fighting, and negotiated a safe retreat in exchange for the nobles in February 1549.
Question: Who invaded Siam?
Answer: Tabinshwehti | Bayinnaung