In OTL, the USSR joined German aggression against Poland on 17th of September, 1939. Although Polish armed forces were trying to resist Red Army's offensive (admittedly, Poles were very weak after two weeks of battles on German front, but, nonetheless, few hundred Soviet soldiers have perished in their "Liberation march"), Polish government had not announced state of war with the USSR. Later, Polish government in exile explained such strange pacifism as mean to ensure free passage of Polish soldiers and civilian refugees through Soviet lines to Romania.
Thus, Western allies of Poland fought only against Germany, although Poland was divided by two aggressors.
So, POD: What if Poland on 17th of September, 1939 declared war against the USSR? Britain and France were obliged to help Poland against every aggressor, and war between those powers and the USSR could be imminent. In such situation Soviet Union had to look for allies, and Reich was most likely ally. The Soviet-German coalition could invade British and French colonies and protectorates in Middle East, and Soviet assistance could greatly increase power of Germans in the West front.
On the other hand, Empire of Japan had suffered defeat in the hands of Soviet army in August, 1939 (Nomonhan incident), so Japan easily could join Western coalition. Thus, we can see World War II without Pearl-Harbor and without German invasion into the USSR.
Any thoughts?