This has been asked before: http://www.reddit.com/r/AskHistorians/comments/1g1kbe/when_hitler_and_stalin_divided_up_poland_why/

Not to detract from any new answers I hope, certainly this question could benefit from a more comprehensive answer than given in the link. I don't have time to answer this right now.

I have another question. I took a class on the Inter-War Period and I remember my professor talking about France and England declaring war on Germany, but not acting, but also began an unofficial war with the Soviet Union without declaring. Does this have any virtue? He claimed that the Nazi invasion of France saved the allies in WWII because France was mobilizing troops in the Mid East to move up the Caucasus and attack the USSR. This in turn stopped major escalations in conflict between the Western Allies and the Soviets. He also said they funded and aided the Finnish in the Winter War.

Does anyone have more insight on this?

However, Germany invaded Poland concurrently with the USSR as the two nations had agreed to partition Eastern Europe between themselves (Molotov-Ribbentrop pact). If the initial declaration of war was to protect Polish sovereignty, why only declare war on only one of the aggressors?

Quick correction: The Invasion was not concurrent.

The Germans invaded on 9/1/39 The Soviets invaded on 9/17/39

This primarily boils down to four main topics: the prospect of fighting both the Soviet Union and Germany, the United Kingdom's pact with Poland, France's pact with Poland, and the swiftness of Polish defeat.

The first is the most obvious: namely, that the Allies in 1939 had absolutely no desire to engage the Soviets and Germans at the same time. While Britain theoretically had the naval power to keep both the Soviet and German fleets bottled up in the Baltic, the combined might of the French and British land armies would have stood absolutely no chance against the combined might of the USSR and Germany. Even assuming the Germans kept a sizable portion of their army in the East, they could still raise an army of nearly 145 divisions. This is an even match for the combined French and British forces available at that time (as demonstrated during the actual Battle of France. This is roughly 3.3 million soldiers per side. The Soviets, despite being largely viewed as militarily incompetent, had a massive army at that time-- of over 5 million soldiers, and could mobilize significant reserves, as the Germans learned to their dismay during the actual war. Furthermore, and this is often missed, the Soviets had the largest armored force in the world at that time. While the Germans had roughly 5,000 tanks, the Soviets had between 15,000 and 20,000.

Diplomatically, things were also complicated. Under the terms of the Polish-British Common Defence Pact signed just over a week before the German invasion of Poland, the British had promised assistance if a European power attacked Poland-- but buried in that agreement was the stipulation (and expectation) that said country would be Germany. Indeed, the idea of a Soviet-German pact to divide Poland caught most everyone off-guard. The Nazis had risen to power by breaking the back of the Communist movement in Germany, after all, and German propaganda at that time routinely depicted the citizens of the Soviet Union as subhuman. The British did discuss pledging publicly to restore the Polish state after the war, but this was eventually decided against-- there remained hope for a USSR/Allies alliance. British (and French) colonial policy would have also been threatened by a war with the Soviets. With most of the Allied forces engaged in the Western front, the Soviets would have been able to invade southward, into the middle east and India primarily, at their relative leisure.

France's situation was more straightforward. The French had signed an Alliance with Poland in 1921, but between then and 1939 that relationship had soured somewhat. When war broke out, the French military leadership were vehemently opposed to assisting the state of Poland directly, and simply refused to widen the war to include the USSR. This ties into my fourth topic as well, as French and British plans were developed upon the assumption that Poland would resist for a significant amount of time. The only offensive made by the French during 1939 was a probing invasion of the Saarland, intended to take pressure off the Polish front. As the Polish army collapsed, Poland began to move towards adopting a new defensive position in the southeast of the country, which would have included evacuating the capital. This "National Redoubt", however, was compromised when the USSR invaded. By then, there was no incentive for the British or French to declare war on the USSR. Poland was lost, and the handful of states left in the east (Latvia, Estonia, etc) were left to the Soviets as well.

All army strengths are rough approximations, pulled from Soviet/German/British/French unit counts around 1939/1940.