So crazy, I just posted this as a question before I really looked through this forum. There’s really nothing fascinating in what I consider to be the “real reason” they only declared war on Germany. Answer, they couldn’t fight a war vs the USSR too, they weren’t even ready for a war vs Germany which was evident when France was overrun in one month. AFTER that event, their only hope was that the USSR (and the USA) would eventually side with the allies to fight Germany, quite possibly the only way they would win the war post 1940. Barbarosa forced the USSR into the alliance, and the rest is history.
Of course the moral implications of this are what we are all really thinking about, the “why?”. As christiaan put it quite well, they didn’t give a damn about the humanitarian reasons, or the lives of the Poles per se, only balance of power and the checks they could provide on any European aggression by being their “ally in the region”. It’s SIGNIFICANCE, to me, is that this type of nation building is still commonplace today.