Also, the UK could actually declare war on Japan UK 1, which would result in them convoy raiding Siam for -1 IPC on Japan’s next turn. However, UK declaring war would not bring USA into the conflict and would allow Japan many more possibilities against UK and ANZAC before the US could help them. Not a good idea just to take 1 IPC from Japan.
Also, in Alpha3 it’s a roll of a dice, so it’s possible that the battleship will roll above a 3 and not even deduct the value of Siam (1).