Upvote:7
The fascist government of Thailand was pressured into declaring war on the Allies by the Japanese, who strong-armed their way into Thailand to build military bases and roads. The Thai government went along, hoping that this would appease the Japanese, who would leave after the war. (You'd think they would have heard of Manchuko or Korea...)
Here is the Wiki article on the Japanese Occupation of Thailand that explains it.
Upvote:13
The Thais had a puppet government that followed the will of the Japanese (such as declaring war on the Allies). That made them nominally, at least, an Axis power. Also, Thailand allowed its territory to be used by the Japanese as a springboard for their invasions of Burma (Myanmar) and the East Indies (modern Indonesia).
Even so, Thailand contributed few troops or other supplies to Japan during World War II. Thus, their "participation" on the Axis side was treated (and viewed) as "symbolic." The role of "Free" (dissident) Thais in resisting the Japanese somewhat mitigated the stigma that was attached to the official Thai government for its actions during the war.