The expansion of the kingdom of Ayutthaya
King U-Thong (King Ramathibodi I) and his immediate successors steadily expanded the Kingdom northwards to include the old kingdom of Sukothai and southwards towards Malaya. However, Siam was constantly under attack by various Burmese kings. After one invasion and after a period of fifteen years in captivity King Naresuan returned to lead his people. Throughout his reign he fought against the Burmese and finally weakened them so they no longer threatened Siam. Then there followed a great period of expansion of the kingdom of Ayutthaya. It became one of the strongest and most prosperous kingdoms in the region.