Bayinnaung had conquered the whole kingdom of Lanna in 1558, with auxiliary troops from Lanna, he led a massive army into Sukhothai Kingdom. Maha Thammarachathirat then realised the greatness of Bayinnaung army and readily surrendered, giving up Phitsanulok and becoming Burmese tributary. At Chainat, Bayinnaung clashed with Prince Ramesuan's army but was able to break through.