Hello! Short answer (at least in WarPlan Pacific) is that I was attempting to transfer the relatively numerous Indian Infantry corps into China.. which apparently isn't allowed? I missed that significant fact in the rules... the Indian army with help from the Americans had taken/liberated Thailand ...

In both WarPlan & WarPlan Pacific as playing as the Allied player in single player games I've run into the same issue - allied units are not allowed to enter hexes controlled by another allied nation??? Why not?? I did not see this issue playing WarPlan (Europe) as the Axis player in a single player ...