Poland wasn't the based state then that it is now. Poland was loooooong an enemy of Russia. I'm forgetting a lot of specifics, but, look at the Polish Corridor/Danzig Corridor whereby Germany had been cut off from the whatever Sea. Poland had been vexing German citizens (I won't go so far as to say atrocities, but, I won't say they weren't, either.) Even Neville Chamberlain had to get onto them, albeit at Germany's request, to knock it off. It wasn't 'Poor Poland, just wanted love and peace and to smoke dope, then, the Meanie Nazis attacked them!" They were doing just as much dirt as anybody else. They just couldn't back it up. And, the whole "Germans killed X, and dressed them in Polish army uniforms and then attacked their own radio stations, so they could start a war!" is so transparently obvious as to be laughable. Note, that that wasn't Germany's motif in gaining land.

So, yes, Germany and Russia DID invade a sovereign nation, A sovereign nation that was fucking Germany hard enough that Germany fought back.