To really understand the USSR's actions during the RA1 timeline we have to go back to the aftermath of WWI. The Russian Empire went through a civil war in 1917 that caused Lithuania, Latvia, and Estonia to gain a brief independence. Armenia, Georgia, and Azerbaijan were also established as independent states until proclaimed as Soviet Republics in 1922 and eventually absorbed into the Soviet Union.
RA1 timeline stuff:
In the 1940s, Stalin took back the Baltic States, some parts of Finland, and the east end of Poland. Due to the fall of the Nazi Party, some eastern parts of Germany are given to Poland--though I'm not a history buff so I can't say exactly why.
In the 1950s, Stalin invades China and Europe, starting the events of RA1 (after Grandenko conquers Asia).
Now, as for the US-Japanese war. Japan's attack on Pearl Harbor had not much to do with Germany's actions. In September 1940, the U.S. placed an embargo on Japan by prohibiting exports of steel, scrap iron, and aviation fuel to Japan because of Japan's takeover of north French Indochina. Soon after Japan's occupation of southern Indochina, the U.S., the Netherlands, and Britain froze Japan's assets, preventing Japan from buying oil.
Japan saw two options: get the oil embargo lifted on terms that would still allow them to take the territory they wanted (Southeast Asia--for more oil), or prepare for war. General Tojo Hideki set November 29 as the final day on which Japan would accept a settlement without war. In order for their war plans to work, Hideki decided that Japan would need to cripple the U.S. Pacific Fleet, hence Pearl Harbor.
In looking at these history, we have to answer some questions:
1) Since the Soviet Union had taken over East Asia, would they have allowed Japan to have the oil they wanted?
a) Perhaps, in exchange for their pledge to the Soviet Union. Though if the USSR had allowed Japan to get around the West's trade bans, how would the West have reacted?
2) Why did some of Germany go to Poland?
3) If Japan was allowed to take Southeast Asia for oil, why did they still attack Pearl Harbor?