They wanted to invade Poland.
Step-by-step explanation:
Germany, ruled by Hitler during this time, wanted to essentially take over Europe and spread Nazism. Hitler wanted to keep expanding and he wanted to expand into Poland as well.
On August 23, 1939, Germany and the Soviet Union had signed a non-aggression pact called the German-Soviet Nonaggression Pact. This meant that they had signed an agreement not to attack each other for ten years.
Hitler signed this non-aggression pact because he wanted to be able to invade Poland without fighting the Soviet Union. He knew he would have to be fighting the forces from Great Britain and France on the west and didn't want his east to be fighting him as well.