In my opinion, WWII was not just caused by the invasion of Poland by Hitler in 1939 but a series of events since WWI.
One of the reasons was WWI, it economically paralysed Europe and it left quite a lot of resentment the various countries. Vast swathes of Europe had been devastated by the war, leaving bitterness on all sides. Also during this war Communism became prevelant in Russia, resulting in a rivalry with the Facist State in Germany in the 1930's.
Another cause in the lead up to WWII was the Treaty of Versaille, forced on the Germans by the victorious allies. The conditions of the treaty were very harsh and forced the once proud nation of Germany to have no more than 100,000 men in it's army. The Germans were also banned from having any war plane, tanks or submarines and from having their army enter the de-militarized Rhineland. Germany was also forced to pay all Britain and France war cost's. Germany also lost 13% of it's land, mainly on the border with France. These land were some of it's most profitable and indutrialised, crippiling it's ability to pay back it's debts. This treaty caused even more German resent towrds the allies, saying that it had been forced upon them.
Because of the treaty, Germany's economic problems worsened. They had lost the Rhur to the French, the industrial heartland of Germany, when they had failed to keep up debt repyments. To keep up the repayments the government printed more bank note, causing inflation to soar. These new bank notes became worthless, you needed wheel barrows of money to buy a loaf of bread and unemplyment rose sharply.
The Nazi's came to power with the promise of more prosperity, jobs and glory for the German People. German's saw Hitler as a saviour, a man who would restore them to their rightful place in the world. Hitler built up the German economy and then broke the Treaty of Versailles by rebuilding the armies and moving them into the de-militarized Rhine-lands, bordering France. In a bid to unite the German people under one banner he said that the Aryan race (Blonde hair, Blue eyes) was superior to all others and that they were the rightful overlords of the other races. Attacks on Jews began resulting in 'Crystal Night', where Jewish shops were broken into and smashed (Crystal) and thousands of Jews were either rounded up or killed. later, when the war was raging, the Nazi's began the final soloution to the Jewish problem....
The League of Nations, which had been set up after WWI, seemed incapable of stopping any wars at all. It was weak and did not act upon it's word. This was seen in 1931-33 when Japan invaded Manchuria in Northern China, to gain natural resources of which Japan had not many. The Chinese appealed to the LoN, the League blamed Japan but the member states did not want to help. More undermining of the League happened in 1935 when Mussolin attacked Abyssina, which is now Ethiopia. The LON imposed sanction upon Italy on all goods except steel, copper and oil - the materials it needed to fight a war. By not acting the LoN showed it's weakness to the rest of the world, including Germany, who now knew that the League would not react to any offensive move they make, and would probably rather make concessions than cross them and then trigger another war.
Another cause in the lead up to WWII was the British and French policy of appeasement towards Germany. Chamberlain believed that by giving Germany everything it wanted, even if it broke treaties, war might be avoided. The German's defied the Treaty of Versailles by uniting with Austria but they allowed it in the name of peace. The German's were also given the Sudtenland in Czechosolvakia to avert war. But all this did was show the Germans that the 2 major powers in Western Europe were weak and would always appease Germany, rather than force war.
But when the German's attacked Poland this was a step too far for the British and French, they did not concede any more and instead went to war for the 2nd time in a Generation, resulting in the deaths of millions of people around the world.
In conclusion, WWII was not just caused by the invasion of Poland but a number of factors steming back to WWI, including the Treaty of Versailles and the weakness of the League of nations.