What happened
The royal chronicles record that in 1351 — traditionally on 4 March — U Thong established a new capital on an easily defended island at the confluence of the Chao Phraya, Lopburi and Pa Sak rivers, and was crowned Ramathibodi I. The city took its name from Ayodhya, the city of the Ramayana.
Background
The area held older settlements linked to Lopburi and the Khmer cultural sphere. U Thong’s own origins are debated: chronicle traditions differ, some making him a lord of the city of U Thong, while other traditions suggest merchant-family origins. One chronicle tradition holds that an epidemic prompted the move to the new site.
Consequences
Ayutthaya became the dominant Siamese state for 416 years. Ramathibodi I established Theravada Buddhism as the state religion and issued early laws, and the kingdom soon pressed against Angkor and absorbed the Sukhothai sphere.