Sukhothai was located on the northern part of the the Chao Phraya River Plain, quite far from Nakornthom, the capital city of the Cambodian Kingdom. Due to the distance, Cambodian political influence over this area was less than that for the southern part of the river, thus opening the opportunity for Sukhothai to establish a Kingdom before the Thai people in the southern part of the river. Moreover, the expansion of Thai people on the northern part of Chao Phraya River and the increase of their political role took place at the same time as the reign of King Chaivoraman, the 7th and last great King of Cambodia. He planned to expand power to the North by fighting against Kingdom of Champa. That war later became long-drawn-out.

Kept busy with warring and at the same time with controlling the land on the southern part of the Chao Phraya River Plain which was strategically significant to Cambodian Kingdom due to its easy access to Yasothorn or Nakornthom city, King Chaivoraman 7th could not completely control the expansion of Thai people's political movement on the northern part of the Chao Phraya River. Left without a choice, he had to support the foundation of a city by these people, as a superior helping the inferior in order to bind close ties between the Cambodian Kingdom and the growing city, which was then considered as the Cambodian Kingdom's dependent. This can be seen from the fact that King Chaivoraman 7th gave King Pba Meung his daughter, Princess Sukaratevi, and a dagger called "Chaisri Pranam Kamoratheng Anyasri Intharabadintratit". Furthermore, if we look at the city plans and water irrigation system, it can be presumed that King Chaivoraman 7th must have taken part in giving assets and technicians to the leaders of the Thai people on the northern part of the Chao Phraya River in order for the foundation of Sukhothai to succeed.

After the reign of King Chaivoraman 7th , the Cambodian Kingdom declined. The political situation was unstable and there was much disorder. The fall of Cambodian Kingdom which was once an important political center of Chao Phraya River Plain created a power vacuum which Sukhothai used as a good opportunity to set up its own kingdom.

Territorial expansion in King Ramkhamhaeng the Great's era depended on the army and on establishing connections among relatives. Since there was no strict control over acquired lands, outer provincial towns were considerably independent and the Kingdom remained integrated solely due to the individual capability of the King. Therefore, after King Ramkhamhaeng the Great, the kingdom split into small groups of towns because his successors were less powerful. During the reign of King Lithai or Thammaracha 1st (1347-1369), Buddhism was used as an instrument to reintegrate all the scattered towns into the Sukhothai kingdom. King Lithai, however, only succeeded in some parts.

From 1378 Sukhothai was dominated by Ayuthtaya for 10 years, but became independent once again in 1388. The battle for the throne between Phraya Bal and Phraya Ram, sons of King Thammaracha 3rd created the opportunity for Ayuthtaya to intervene and redominate Sukhothai. This time, Ayuthtaya separated Sukhothai into 2 parts: one had Pitsanulok as its new capital and the other had Kampaengpeth as the center.

King Rachatirat 2nd appointed his son Prince Ramaesuan whose mother was a Sukhothai royal, viceroy and governor of Pitsanulok. Sukhothai Kingdom was therefore completely annexed to Ayuthtaya Kingdom. This marked the end of Sukhothai Kingdom.