Well basically Adolf hitler wanted more land, especially in the east, to expand Germany according to the Nazi policy of lebensraum.
The British told Germany they have no rights to invade these countries, but Hitler used harsh limitation such as the Versailles Treaty as a pretext for Germany's right to acquire land where German-speaking people lived.
In the eighteenth century Poland was an independent country with its own monarch. Constantly attacked by powerful neighbours it was eventually divided up between Russia, Austria-Hungary and Germany.
In August 1939, a group of concentration camp prisoners were dressed in Polish uniforms, shot and then placed just inside the German border. Adolf Hitler claimed that Poland was attempting to invade Germany. On 1st September, 1939, the German Army was ordered into Poland.