Neutrality was not an option because Thailand was in Japan's way. It would have been difficult to invade Malaya and almost impossible to invade Burma without going through Thailand. The Japanese invaded by sea and by land after Pearl Harbor and the Thais resisted for a few hours. Then they concluded a ceasefire and negotiated an alliance.

Without knowing how the war would end, it made sense for Thailand to ally with Japan. This seemed to spare it from a war fought on its own territory (until the aerial bombings started) and provided the opportunity to annex parts of British Burma and Malaya (which they did) and, should they play their cards right, French Indochina (besides the border regions they obtained in early 1941 after a brief war with the French colonial forces) and southern China. Thailand was going through an irredentist fever at the time, one the Japanese could soothe better than the Allies.