I've asked on this question at several Universities and it seems no one really  knows.

Britain and France declared war on Germany (rightfully so, non aggression pact) when Germany invaded Poland.  Russia invades Poland, no declaration of war by France of Britain violating their non aggression pact with Poland which they used to justify the declaration of war on Germany.

My guess is either their collective hatred  of Germany over Russia/Franco war and WWI or communist influence in British government.  Can't say a two front war, think what that would look like Russians transmitting German soil to get at the Brits, NOT.

Ideas anyone?

On 17 September 1939, the Soviet Union invaded Poland from the east, sixteen days after Germany invaded Poland from the west. Subsequent military operations lasted for the following 20 days and ended on 6 October 1939 with the two-way division and annexation of the entire territory


Do you really think it would have been smart to declare war on Germany and Russia?      Tell the truth.

The French and British did plan military retaliation against the USSR early in the war.  They wanted to send an expedition to Finland to fight the Russians, but the cowardly Vikings wouldn't let them transition Scandinavian territory.

They also planned a strategic bombing campaign against the Caucus oil fields but Adolf invaded France before it get started.

They enemy of my enemy concept.  Britain believed Germany and the Nazis to be the more immediate threat and even though the Russians were not overtly fighting the Germans in 39, there was still quite  bit of the German military tied down in the east.

The alliance or promise to defend Poland was only against Germany, not Wussia.

The Allies also figured that Germany and Wussia would be at each other's throat and all they (Brit & France) had to do was outlive the fight.

It doesn't help Britain's cause that Neville Chamberlin didn't realize that Hitler was just as evil as Churchill predicted until it was too late.  

Churchill called out the Bohiemian Corporal years before 9/1/1939

The defense treaty stipulated that the UK would aid Poland in the event of German aggression. The treaty did not technically cover Russia.  The USSR invaded Poland with the pretext of defending Ukrainians and Belarusian within Poland. Outwardly it appeared as though the Red Army was preventing the Germans from taking over the whole country.  England and France concluded that it was better to treat the USSR as a potential ally.  It would be impossible to liberate Poland when surrounded by two belligerent powers.

Although, British and French military planners made active preparations for a War against the Soviet Union following that country's invasion of Poland in September 1939 and the Invasion of Finland November 1939.
The British plan (Operation Pike) was to bomb the Soviet oilfields, principally at Baku and Batum in the Caucuses region. Approximately five squadrons of RAF aircraft were transferred to the Middle East, with the plan of operating them from airfields in Iran. Britain also conducted high-altitude photo-reconnaissance missions over the targeted areas in early 1940. The French moved a similar number of bombers, basing them in Syria, which at the time was a French protectorate.
As things worked out, the German invasion of Western Europe that commenced on May 10, 1940, made these plans irrelevant. The allies quickly realized that they would need all their available resources to combat the Germans, and that they certainly lacked the resources to enter into another war with a great power.

Great Britain also did not declare war on Slovakia.  Slovakia assisted the German invasion of Poland by allowing the Germans to use its territory as a staging area and assigning 3 Slovak Infantry division and a motorized unit to German Army Group South.   In response to the Slovak actions both UK and France broke off the diplomatic relations with Slovakia.  

Also, Poland did not declare war on the Soviet Union.

In addition to what's been posted above, both the French and British governments (correctly IMHO) saw the Soviet invasion of Poland for what it was - an opportunistic grab that was largely due to their own actions in Czechoslovakia that convinced the Soviets that the West wasn't really serious about stopping Hitler.  While he certainly wasn't happy about it, even Churchill admitted that the Soviets were, to a great extent, pushed into the German camp by the UK and France.   Even the attacks on the Soviet Union mentioned by previous posters were, very significantly, aimed at targets that would, in theory, reduce German imports of iron ore and oil.

The period of time you're referring to is called The Phony War. It's also known as the Sitzkrieg. Much posturing and limited action by the allies.  They really weren't on a war footing at a national level.

I'm still trying to understand how approximately  350,000 of them got ran out of France in 1940.

This is a question I’ve asked many times but most people who aren’t history majors don’t even seem to know that it was France and Britain that declared war on Germany. I’ve even had people argue with me, so confident in the Hitler wanted to conquer the world BS that their public school education fed to them.
My understanding is that there were a lot of business interests in Western Europe that wanted a weak Germany to exploit and Poland was just the point they realized Hitler wasn’t going to be controlled.
Lots of the history surrounding the German invasion of Poland has been revised post war. The BBC documentary 1945: A Savage Peace is worth checking out, basically outlines the genocide of ethnic Germans in Poland to promote the idea that western Poland was never German.

Could go on for hours in the topic. Sorry for bumping a year old post.

well those are all the usual and valid reasons that have been written

but 80 years on I propose more simple solution

commies gonna commie

Britain's state dept was full of commies, we know this now after the war.   USSR is commie.   And Britain definitely went socialist after the war.

and now?  current day Russia is way less commie and corrupt than modern not so Great Britain.

The bigger question to me is why France and Britain sat on their ass while the bulk of the German army was in Poland.

France absolutely did every thing in it's power to not have another war on it's soil.  Hence the sigfried line.  Both Britain and France weren't really up to par with the Germans militarily. So neither really wanted to actually go to war, it was an agreement, and they didn't think Hitler would actually do it.

Britian did a little bombing, France waited for Germany to come through Belgium.  Meanwhile Poland was over run.  Once the Polish Gov't was killed and/or exiled, it literally ceased to exist as a country.  England pretty much said "well, that gets us out of that contract"  When Russian invaded Finland, Finland asked for help, and Britain was All In.  the Fins knew it was bullshit, and took care of it themselves.

I've been reading 'All Hell Let Loose' by Max Hastings.  It is a fascinating book so far, covering a lot of this 'lesser known' stuff like attitudes of the normal people and some of the SNAFU bullshit that was going on early in the war. UK and France were so determined to avoid another WWI that they pretty much made another WWI happen. Like, UK was in full Blitz Lockdown all through the fall and winter of '39, even though Germany literally didn't give a shit.  It reminded me of Covid a tad.  People losing jobs, homes, education, economy getting trashed all cause 'Ze Germans are coming!' even though the gov't knew full well they weren't.  Course, they let it all go back to normal and then boom, The Blitz actually did start.  Kind of like how Dunkirk wasn't exactly this Heroic Last stand, it was more of a ...shit we don't really want all those troops there, get them out of there and the Germans noticed THAT movement and was like..oh well then.

Imagine how things would have gone had Churchill gotten in sooner!