The official policy of the U.S. Government is that Thailand was not an ally of the Axis, and that the United States was not at war with Thailand. The policy of the U.S. Government ever since 1945 has been to treat Thailand not as a former enemy, but rather as a country which had been forced into certain actions by Japanese blackmail, before being occupied by Japanese troops. Thailand has been treated by the United States in the same way as such other Axis-occupied countries as Belgium, Czechoslovakia, Denmark, Greece, Norway, Poland, and the Netherlands.
What is the UK to the US?
an ally