Shorlty before that, the future Allies - Britain and France - colluded with Germany in division of Czechoslovakia (Munich Agreement), and Poland grabbed a piece of the cake, too. But of course that doesn't count. /s
Not everyone knows this when talking about Poland in 1939, but by the 17th of September, when Soviet troops entered the eastern Poland (or western Ukraine, western Belarus and Lithuania's Vilnius region as they are today), the Polish army was already broke. Most of the Polish troops were deployed to the west to fight the Germans, and the Soviets barely faced any resistance on the east. Plus, the commander of the Polish army, Rydz-Smigly, ordered his troops to not resist to the Russians (except for self-defence), and the goverment of Poland was about to run away to Romania. Because of the little resistance, the Red Army was advancing so fast that the supply units weren't able to deliver fuel in time.
So, I don't think it's worth being in a game like Battlefield. Or even the Polish army, that didn't exist for too long during the WW2; the Polish forces that fought for the Allies later were de-facto parts of the British (Anders' army) or Soviet (Berling's army) forces.
On a cite note: before the Polish campaign, the USSR sided against Germany and Italy in the Spanish civil war, and fought Japan twice, who wasn't an Ally by ANY chance.
Like one guy on a Russian forum said, "Please, no USSR in this game! Or we will find out that there were black transgender cyborgs in the Red Army who invaded Poland on IS-3's!".
What do DICE mean by the "own faction"? If they mean the USSR can't fight on the same maps as the western Allies, that is fine (and that's how it should be), but normally a "third faction" means that it can fight against both of the other two. Which weird to say the least.