Which has little or no international standing at the time. At the Algeciras Conference of 1906, Italy basically had to beg an invite despite being a nominal ally of Germany while the US had been invited as an official observer from the first.
Getting back to the Treaty of Portsmouth, both nations were in desperate straits. While Russia was facing a slow motion revolution, Japan was essentially bankrupt, her economy imploding, and her reserves exhausted. Sergei Witte understood this and he played a very tight game at the conference. He bet, correctly, that Japan would have to blink first and presented Japan with a series of take it or leave it decisions.
The US didn't talk Japan into accepting one offer or another and the US didn't talk Russia into making one offer or another. The decisions made by both parties were almost entirely determined by domestic political issues. Japan chose de jure recognition of her de facto control of southern Sakhalin over an indemnity which Witte had deliberately kept low. When the terms of the treaty were published in Japan, it was the lack of an indemnity and economic boost it would have created that sparked condemnation and riots and not the "loss" of northern Sakhalin.
Finally, the suggestion that gaining Sakhalin would somehow damping or short circuit Japanese militarism is asinine. Japan's militarists had a long list of sleights, snubs, and provocations, both real and imagined, of which the "failure" to gain the whole of Sakhalin is just one small entry. The Unequal Treaties, the Three Power Intervention, US immigration policies, Western support of China, and others were already sources of humiliation while Western reaction to the 21 Demands, Versailles, the Washington Naval Treaty, and others will be sources of humiliation later.
Sakhalin is just a drop in a bucket already filled.