This is not to discourage further questions, but since the first hit below was asked 4 days ago, you might ask them there for one-stop shopping.
There is a search function: on the top in the Android app, at upper right on the Web. I did a search for
france poland
It pointed to In 1939, when Germany and USSR invaded Poland, Britain and France reacted by declaring war on Germany. Why did USSR get ignored?, in which /u/Georgy_K_Zhukov updated and expanded his usual answer.
Another current usual answer is from /u/kieslowskifan, which I last found from 1 month ago in Why did the UK & France declare war on Nazi Germany but not the USSR during the invasion of Poland?.
There's also Why did France and the United Kingdom declare war on nazi Germany in september 1939 for invading Poland but not on the Soviet Union for doing exactly the same ?: /u/QuickSpore gave a shorter answer with much of the same information, but in a grandchild comment gave a couple of other sources that do not intersect with /u/Georgy_K_Zhukov's sources.
At a quick scan, they seem to be pretty much in agreement: the guarantees to Poland were only given against a German invasion, the UK in particular thought that the USSR's seizures had some justification, and the UK + France had hopes of a USSR-Germany falling out. But that was just my quick glance; you might like to read more in depth and correct me if my impression was wrong.
You can get to the FAQ from the right sidebar on the Web, or Community Info in the Android app. The main FAQ page has a section World War II, with a link Soviet Union, with a link Soviet-Polish Relations, with