Why was Poland invaded?

Which invasion? Poland as a country has a history of over 1000 years and has been invaded many times. The most infamous recent invasion was in 1939 when Nazi Germany invaded. The reasons for this invasion are numerous but are often given that Germany needed more space for Germans to live in and saw their eastern neighbours as sub-human. Germany also wanted to remove Jews and other ethnic groups which they hated. The explanation for the actual invasion is often given as a retaliation because Polish troops attacked German troops on the border but this has long been disproven and the Germans made it look like they had been attacked first by the Poles.