The invasion of Poland by Nazi-Germany on the first of september was the tipping point when the allied nations realized that the appeasement policy did not work. As a result they declared war on Nazi-Germany with the reason that it was an invasion of an independent and sovereign country which it was. Yet 16 days later the SU invaded Poland as well on the 17th of september but this time the allies did not declare war on the SU. My question therefore is quite simple: Why did they not?
Did they realize that Poland could not be saved anymore? Were they afraid that the SU and Nazi-Germany would team up against them?
P.S. if there are spelling errors or other minor issues (which there probably are), excuse it as it is not my mother tongue.
It's a rather common question, and there's an answer in the FAQ: https://www.reddit.com/r/AskHistorians/comments/agimfc/in_september_1939_germany_and_the_soviet_union/ee6y4bb/ by /u/Georgy_K_Zhukov
At least two, in fact:
https://www.reddit.com/r/AskHistorians/comments/3b3g0t/why_did_england_and_france_only_declare_war_on/csixa8k/?context=999 by /u/Balnibarbian
For a short summary, unless something new has come up, the general gist is that the Allies guaranteed Polish independence. The fact that it was an invasion of an independent and sovereign country was largely irrelevant, the real reason was that there was this specific guarantee. There was a secret protocol which was even more particular about the guarantee being specifically targeted at a German invasion, rather than a blanket guarantee.
The linked posts go far more in depth and phrase it a lot better than I probably could, however. I strongly recommend checking the FAQ out, it has some great material.
There's a lot of varied takes on this question, each and every one of them worth a read: