How a major Siamese fleet changes the war for Southeast Asia

What changes if Siam receives a navy equal to seventy percent of the Imperial Japanese Navy in 1941?

Siam's choice of alliance becomes decisive: it could greatly strengthen Japan or help prevent Singapore's fall, while its very existence would reshape Japanese treaty and threat calculations.

A force this large could include six or seven battleships and two or three carriers, making it a regional navy that neither Japan nor Britain could ignore. If Siam aligned with Britain, its fleet and the alert prompted by Japan's move into French Indochina could make the fall of Singapore far less likely. If it aligned with Japan, the same ships would greatly intensify the threat to British and American positions. Its existence might also encourage Japan to leave the naval treaty system earlier because Tokyo would see another large neighboring fleet as a direct constraint and potential enemy.