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Wow, this is a great question. There are numerous reasons some of the them have more validity than others. Here are a few:
Great, thanks so much for the help already.
1. To give Germans lebensraum in Eastern Europe He had promised this in Mein Kampf (1924) and it was one of the three CENTRAL AIMS of Hitler foreign policy. 2. Because he thought Chamberlain would not dare stop him Chamberlain had stood up to Hitler, remember, at Bad Godesberg during the Sudeten crisis, but had then backed down at Munich. Hitler despised Chamberlain, and did not believe that he would dare to go to war. So he felt able to pursue his aims in Poland despite Chamberlain's promise in March 1939 to support Poland. 3. To defend the Germans in Poland The reason Hitler gave was that the Poles were persecuting those Germans who lived in Poland. (There was some truth in this.) 4. To overturn the Treaty of Versailles This was a second CENTRAL AIM of Hitler's foreign policy. The Polish Corridor and Posen had been given to Poland in 1919, and Danzig had been declared a free city administered by the League of Nations. Hitler first asked Poland to consider the position of Danzig in October 1938, immediately after Munich, and in March 1939, Hitler demanded that he be given Danzig (this was the pattern he had followed with Austria and the Sudetenland). Did you know that in March 1939 also, Germany seized the Lithuanian port of Memel (at the northern end of East Prussia)? When Hitler demanded Danzig in March 1939, Brauchitsch, the Commander in Chief of the German Army noted that he intended ultimately to 'knock Poland down completely', and that eventually Hitler wanted Germany's pre-WWI boundary restoring. 5. To oppose Communism/conquer Russia I know Poland wasn't communist, but Russia was where Hitler was eventually headed (Mein Kampf, 1924) and Poland was just another step east. When he demanded Danzig in 1939, Hitler's proposal included a joint anti-Soviet alliance against Russia. This was the third CENTRAL AIM of Hitler foreign policy. 6. To teach Chamberlain a lesson Chamberlain's guarantee of Poland on 31 March 1939 infuriated Hitler - 'I'll cook them a stew they'll choke on' - was his reaction. From then on he was determined to destroy Poland. So you could say he wanted to attack Poland to teach Chamberlain a lesson. 7. To prevent an anti-German alliance Having thought about it, he realised also that the world was beginning to gang up on him, so the next day, 1 April, his CONSIDERED reaction was this: 'if they expect Germany to sit patiently by while they create satellite States and set them against Germany, then they are mistaken'. This is fair enough, actually, because that is exactly what Chamberlain was trying to do. And Poland was preparing to resist Hitler, and had started mobilising its army - Hitler stated that this broke Poland's non-aggression pact with Germany [see note below]. On April 3 Hitler issued a directive to his armies - entitled 'Case White' - stating that he wished to 'destroy Polish military strength and create in the East a situation which satisfies the requirements of national defence'. In this document, he set the date for 'Case White' - 'any time from 1 September 1939 onward.' - and told the Werhmacht to draw up a timetable. 8. The Nazi-Soviet Pact After April 1939, both Roosevelt and Stalin began to express concerns about Hitler's aims on Poland. Hitler merely mocked Roosevelt, but he was worried about Stalin. Only Stalin - and the Russian army - could have stopped Hitler taking over Poland at this point. But the failure of the Anglo-Soviet negotiations and the Nazi-Soviet Pact of 23 August 1939 not only freed up Hitler to attack Poland, it included a secret agreement to divide Poland up between them. In the end, Hitler invaded Poland because he had agreed to do so with Stalin.
Hitler had planned to invade Poland with the USSR but then he wanted to take over by himself so he went ahead and claimed it by himself. Poland wasn't claimed by anyone yet.
Hitler claaimed that it was retaliatorey act because of anti-German Polish aggression. Of course the Nazi-Soviet pact signed 8 days previously to the invasion suggests something altogether more complicated. Think Suddetenland, he did not need any help there.