Steve is correct.
But, moreover, the situation with Polish-German relations let alone Polish-European relations of that era is one that's largely overlooked today in history, unfortunately, by everyone.
The Poles had gone to the League of Nations to get them to award their country what parts of Silesia Germany had retained in 1921 (which was done), they had delivered an ultimatum to the Central Republic of Lithuania that demanded they hand over their territory and dissolve their government or risk war (to which the much smaller CRL complied) in 1922, and then they went back to the League of Nations in 1924, complaining about their border with Czechoslovakia (much as they had done with Germany three years previously)- after which they were awarded most of Nowy Targ County (the Czechs demanded beforehand that if this were to come to pass, that they receive in turn Sucha Gora and Glodowka in return, and this was done thankfully).
http://en.wikipedia.org/wiki/Territo...of_Poland#1921
http://en.wikipedia.org/wiki/Territo...of_Poland#1922
http://en.wikipedia.org/wiki/Territo...of_Poland#1924
The Germans had initially been in the 1930s not so interested in taking over all of Poland as part of their foreign policy as much as they had been interested in taking back the provinces which had belonged to them (including Silesia, parts of Pomerania, the provinces along the Lower Oder River Valley, etc.). Primarily, they were focused on Danzig, because it was the province that would connect the heartland with their provinces that made up East Prussia (Konigsberg and Elbing) and because it had a HUGE ethnically German population (ethnic borders comprised a large part of their foreign policy's focus, just so you know- Austria and Czechoslovakia in particular).
http://upload.wikimedia.org/wikipedi...ungsgebiet.jpg
So initially, they asked the Poles for Danzig. They said no. Then they pointed out that they had been awarded Silesia on the grounds of ethnicity, so it was only fair that Germany receive Danzig on the same grounds. The Poles still said no. The people of Danzig, however, finally said that they did not want to be a part of Poland; they wanted to be made a part of Germany.
The Poles responded by occupying them with several garrisons of troops, and by suppressing the government that was in place there (did I mention that Danzig was a self-governing city-state that had its own republican form of government- run by a President of the Senate in the Volkstag?).
http://www.zum.de/whkmla/region/eceu...g19191939.html
In August 1939, Joachim von Ribbentrop was sent to discuss the feasibility of getting the Soviets to help Germany invade Poland if they refused a final time to submit to Germany's demands. This decision was made around the theory that, since the operation would be in conjunction withe the Soviets, the United Kingdom and France (both of whom had guaranteed they would support Poland if an invasion by Germany took place) would not seek to protest the issue with anybody or hold up to their promises to the Poles, risking war (if they did) with both Germany AND the Soviet Union (and even assuming they did, the Germans knew then, at least, they would have security in the east so the bulk of its military could be devoted to beating the French and British in the west).
http://ww2db.com/battle_spec.php?battle_id=90
And the Soviets agreed, largely because they were pissed at the Poles for the war they'd fought with them back in 1919. Hence, the Molotov-Ribbentrop Pact came to existence in the late hours of August 23rd. The next few days saw Germany asking Poland one final time for Danzig and the other respective border provinces it wished to have, the British and French still voicing their support for the Poles, lots of general mobilizations of troops, and finally, on September 1st, war.
The rest is pretty straightforward. Poland was divided between Germany and the Soviet Union, and the British and French kept their word and declared war on Germany (this really is when the Second World War started- when three major world powers and numerously smaller nations that were a part of the British Commonwealth, such as Australia, Canada, Iraq, etc., faced off against one another).
Initially, the Germans made no attacks on the French or British on land, because the French and British made no attacks on them (see the "Phoney War"). This was largely in part due to the fact that the First World War's effects were still being felt in both countries, even 20+ years on. They had both become, as Hitler wrote,
"politically and militarily weak".
But there was another reason: the Germans had not originally planned to go to war with either nation. In fact, they had originally dreamed of "sharing" Europe WITH them (they'd even gone through a period where they tried to be like the English in their mannerisms and they tried to be more culturally influential like the French, particularly with music and art), considering instead to expand eastward into the resource-rich areas of Russia.
Of course, in the end, it never worked out that way. Because of the outlook on Versailles and the outcome of the First World War, it was believed by most Germans that a ground war WITH the French and a decisive defeat on the English should be launched. And, in 1940, that's exactly what happened: it all blew up. The British and French decided that it was time to start thinking tactically about what countries it should concentrate on working with in order to beat the Germans, the Germans, in conjunction with this and their aforementioned feelings about Versailles, decided that it was time to fight back.
http://www.worldwariihistory.info/1940.html
The remaining five years of conflict are pretty self-explanatory. The end result of it all:
78,878,170 people died. There were lots of things that should have been done in the beginning that weren't, by both sides, that ultimately led to the war. The Poles should have given over the territory that rightfully belonged to Germany one of the two times it was requested, the British and French should not have decided to support such a minor country as Poland and should not have declared war on the Germans following Germany's invasion to take what belonged to them, and the Germans should not have decided to go on the offensive against the British and French in 1940.
Although the remaining five years, let alone this period of a few months, were vastly more complex than this brief explanation... ultimately, World War II was just as preventable as World War I. And it probably would have been for the better if it had been prevented.
EDIT:
One thing's for certain: 78,878,170 people would not have died.