evidence: Holding the northern part of Sakhalin (Karafuto) do also give Japan a substantial oil deposits, although most of it is in shallow sea, realistically preventing development until after WWII. And Japan is still very likely to lose Sakhalin in closing stages of WWII. [...] The Americans would probably accept a Soviet annexation of Sakhalin if it happened quickly enough at or near the end of the war, even though it would have clearly been a core Japanese territory by this point. Remember that South Sakhalin/Karafuto was already pretty well integrated with the Japanese home islands IOTL but this did not prevent Roosevelt from signing it away. Its population in 1945 would not be much greater with control of North Sakhalin because of the very harsh climate in the Northern half of the island. What really works against it though is the fact that Japan gained it in war in the first place and so the Americans, temporarily still in a Pacific war mindset and eager to suppress militarism, will be much more hesitant to agree that it constitutes core territory.