I'm playing Poland right now and doing really well. (note: Not playing historical)
Conquer Romania first, then Hungary. Romania is an easy first pick and no one will justify at 0 world tension. Hungary is Fascist, so same thing.
Form a faction with the Baltics, and pick up Yugoslavia, Finland, so on. They'll be super terrified of you and give in pretty easy if you're not at war.
Now the hard part here is avoiding a Communist + Axis war that will destroy your little neutral faction. I ended up conceding some territories to the Germans to delay them until they went to war with France, and then attacked them when they were fighting the Allies for a 2 pronged war. After crushing the relatively weak early Germans I was able to turn my attention to the Russian bear as it woke up. Italy was always a thorn but the allies eventually tore them up.
Russians are good on defense, bad at offense with their gazillions of infantry. I managed to stall them out, with only 200k casualties to their 3 million. With a democratic coup in their backside it was an easy counter push to Moscow (make sure to have lots of tanks). The Russian federation ended up taking over, so my brief instant of a Poland empire immediately vanished, but nevertheless the Communist threat was gone. Now I sit here, wondering why I'm at war with Japan if I can never reach them and why there's no way to offer them a peace treaty. I'm also wondering how the spoils of war are divvied up since both my faction and the allies are at war with the Axis.