In the early Thailand years, before the first capital, there was no country of Thailand. It was a collection of city states each with their own kings. There were no defined borders, and they were more like governed islands surrounded by a sea of ‘international’ land. Languages of each region were very similar, like the regional Thai dialects today are very similar. And on occasion, these city states even went to war against each other.
Sukhothai (approximately from 1238 – 1378AD)
Sukhothai, which literally means ‘Dawn of Happiness’, was the capital of the Thai Empire for approximately 140 years. The Thai alphabet was developed during this time. At this time, Sukhothai and Ayutthaya both simultaneously existed as different kingdoms. But in 1378 Sukhothai lost to an invasion from Ayutthaya, they were united in 1448 from a royal marriage, and finally annexed by Ayutthaya in 1583. See a map of Sukhothai in 1300.
Ayutthaya (approximately from 1350 to 1767)
Ayutthaya was a huge prosperous trading city which eventually was sacked and burned by an invading Burmese army in 1767. Ayuttaya was a warring state, and had militarily conquered many surrounding regions, including Sukhothai, Chiang Mai, Angkor (of Cambodia), Laos, Malaysia, etc. Burma and Ayutthaya had repeatedly gone to war with each other, with neither being successful at conquering the other until 1767. Fortunately for Thailand, Burma was also at war with China and had to retreat it’s forces not much later to save itself.See a map of Ayutthaya in 1400.
Thonburi (1767 to 1782)
The details are fuzzy, but after Ayutthaya was sacked and the capital was moved to Thonburi, King Taksin apparantly was becoming mentally ill. Or, at least that was the reason used for the coup that over threw and executed him to install Rama I. King Rama then moved the capital to Bangkok. Supposedly Thonburi also had flooding problems which was a major factor for the move. Thonburi even today still has flooding problems.
<a href=http://en.wikipedia.org/wiki/Thonburi>Learn more about Thonburi at Wikipedia</a>.
Bangkok (1782 to present)
The advantage of the capital being located in Thonburi and later Bangkok was both for militarily strategic reasons to defend against the Burmese, but also closeness to sea trade ports. There was talk in 2012 about moving the capital again for flooding reasons. The city is in the middle of a flood delta and the land is slowly sinking below sea level.