I'd like to add to /u/kieslowskifan's answer by pointing out that the declaration of war against Germany really had very little to do with a desire to defend Poland (and that, in fact, neither France nor Britain came to Poland's defense after declaring war).

Technically, France had no obligation to come to Poland's defense against the Soviet Union while Great Britain did. In 1939, both nations had negotiated a defense pact with Poland, though the agreement between Poland and France specifically named Nazi Germany as a potential aggressor whereas the British agreement referred to "hostilities with a European Power." But whether one nation technically had an agreement with Poland really did not matter. What mattered was the history that led up to Britain and France's declaration of war against Germany. In 1938, after Germany annexed Austria, Hitler demanded that the ethnically-German portions of Czechoslovakia also be transferred to his control. Czechoslovakia, under president (and former foreign minister) Edward Benes, had feared such aggression from Germany since Hitler's rise to power and so the nation had negotiated a series of defense treaties during the 1930s aimed at containing German aggression. Most importantly, Benes had worked out a mutual defense pact with France. When it looked as though Hitler would be willing to go to war over the issue and Britain showed no desire to back France in a war, however, Paris reneged on the deal and sided with London in a move to appease Hitler. At the infamous Munich Conference of 1938, the leaders of France, Britain, Germany, and Italy (but not Czechoslovakia) agreed to transfer portions of Czechoslovakia to Germany with assurances from Hitler that he would not seize any other portion (i.e., the non-German parts) of Czechoslovakia. Neville Chamberlain returned to London famously touting that the Munich Agreement have preserved "peace in our time." Czechoslovakia, abandoned by its allies, had no choice but to submit to the deal.

As odd as it may seem in retrospect, this "appeasement of Hitler" was seen around the world as a great moment in European diplomacy. The Secretary of State of the United States Hull called for European powers to "redouble" the efforts made at Munich in the case of future crisis. This all changed dramatically in March of 1939 when Hitler occupied the remainder of the Czech portions of Czechoslovakia (which became the Protectorate of Bohemia and Moravia) and worked with Slovak nationalists to create a pro-Nazi Slovak state. At this point, it became clear to policymakers around the world that Hitler's ethnically-based demands had really been a ploy to expand the power of the German military state. He now had in hand the industry of Bohemia, which, in the case of a potential war in Europe, would now be used to boost the German war effort rather than counter it. The appeasement of Hitler had only "whetted" the appetite of the aggressor (or so the Lesson of Munich informed world leaders).

Thus when Hitler began demanding that the Polish Corridor separating Germany from East Prussia be ceded to him, France and Britain had no choice but to face the harsh reality that Hitler intended not to stop with this demand (as he once again promised) but to continue to expand the German war machine until Germany became unstoppable. Thus in the wake of Hitler's invasion and dissolution of Czechoslovakia, France and Britain promised that (unlike the case of Czechoslovakia) the two powers would go to war to protect Poland. Of course, neither power actually wanted war with Germany, but this was the reality that they faced: either guarantee Poland's existence, or watch as Germany continued to grow into a superpower. Thus when Germany finally did invade Poland, both powers felt they had no choice but to declare war.

It should be noted, however, that Hitler believed both France and Britain (especially Britain) would back down even after declaring war. Indeed, except for one small, abortive incursion into Germany territory by the French army, neither France nor Britain actually came to Poland's aid. Neither France nor Britain was prepared to do so. Some analysts have called this a second betrayal of a Slavic ally (after Czechoslovakia), but in any case it was certain that Britain and France weren't going to followup one empty declaration of war with another against the Soviet Union whether they had formal defense pacts with Poland or not (Britain did).

And thus Europe entered into a strange period of the Second World War known as the "Phoney War." During this period, France and Britain remained technically at war with Germany and refused to negotiate a peace with Hitler, and yet neither power attacked Germany directly (there was military action in Norway, which Hitler occupied, as well as naval engagements). The Phoney War ended suddenly in May of 1940, a full eight months after the invasion of Poland, when Germany invaded France and the Low Countries.

The long and the short of it is that Anglo-French commitments to Poland only pertained to German aggression. The timing of the Soviet invasion threw Allied foreign policy into disarray with some in the French and British government and public sphere calling for anti-Soviet actions up to a declaration of war, and others adapting a wait and see approach hoping for the German-Soviet rapprochement to collapse.

Over the winter of 1939/40, advocates of a military response against the USSR gained some traction in light of the stalled Soviet war against Finland. There was some preliminary French military planning for airstrikes against the Soviet oilfields of Baku and in December, the British war cabinet authorized the sale of military equipment to Finland. But plans for a more substantive military intervention never really got off the ground. Range limitations of the current generation of RAF and Armée de l'Air bombers made Middle East-based bombing of Baku quite difficult. The British cabinet was quite split over Baku, with hawks like Churchill favoring bombing or the threat of bombing to cow the Soviets into not making oil deliveries to the Germans while Chamberlain feared an empty threat would only prompt the Soviets to beef up the Baku air defenses. Soviet breakthroughs in Finland likewise rendered military assistance to the Finns moot.

One of the important dimensions of these anti-Soviet plans was that they were designed as a form of economic warfare against Germany. Even intervention in favor of the Finns had an important component of trying to sway neutral opinion in favor of the Allies. So while attacking the USSR's aid to Germany was militarily out of reach, an intervention in Scandinavia was not. Again like in Finland, facts on the ground put an end to these schemes as the Germans simultaneously anticipated a Scandinavian operation. There was some debate in London and Paris as to whether after Scandinavian they would be in a better position to threaten the USSR with real or feigned military intervention to stop its supply of raw materials to Germany. The Norway debacle and the Fall of France rendered this debate moot since Britain would have to prioritize using its armed forces against Germany and her Italian allies.

Anglo-Polish common defense pact of 1939 didn't oblige GB to declare war on USSR if they would have attacked Poland. Precisely both sides declared mutual assistance in case of attack by one of the "European powers", but in the secret agreement has been added that agreement works unconditionally only in case of german attack.

Franco-Polish alliance of 1921 also was an exclusively anti-german. Only anti-soviet polish alliance was an alliance with Romania. Both sites agreed "to assist each other in case one of them was attacked, without giving a reason for his part, on its current eastern borders".

USSR never officially declared war on Poland. Soviet forces marched into Poland under the pretext of protecting rights of Belarusians and Ukrainians living on Eastern border of Poland. Also soviet ambassador stated that polish state had collapsed and due to this fact it no longer could protect its own citizens, of course it wasn't true at that time. Moreover it was an agression not only de facto but also de iure, since USSR and its european neighbors signed the Conventions for the Definition of Aggression in 1933. The convention had defined an act of aggression as an invasion by one country armed forces, with or without a declaration of war, of the territory of another State.

So as we see theoretically only Great Britain and Romania had formal obligations to declare war on USSR. France did not recognize that soviet attack on Poland was an assistance in German aggresion, in other case they would have been obliged to declare war on USSR. Same situation was with Great Britain. But as we know Great Britain did not have to declare the war since alliance with Poland worked unconditionally only in case of war with Germany. We could say governemnets of GB and France used all legal possibilites to avoid war with USSR. As regards to Romania polish government never asked Romania of assistance in war with Soviet Union, because Romania probably wouldn't decide to declare war on USSR and polish governement needed romanian territory as a safe passage.

Source: Henryk Zieliński, Historia Polski 1914-1939

Is anyone able to answer further especially concerning Finland? Making a new question seems a bit redundant. I'm Finnish and even our own history courses in grade school and high school don't strive to answer this question, even though Winter War is seen as a major point in our national history.

I apologize if this post is in the wrong place/otherwise inappropriate.