How did the German invasion of Poland lead to a wider European war?
The German invasion of Poland and the occupation of eastern Poland, by agreement in the German-Soviet Non-aggression Pact, caused alarm in England and France. After a few days England and France declared war of Germany. As part of its agreement with Germany, Italy began military operations against the British in the Mediterranean Sea. And by November of 1939 the USSR had invaded Finland.
Germany had already taken over Czeckoslovakia and Austria. Hitler had managed, by a brilliant combination of threats and diplomacy, to achieve these without war. When he remained beligerant and it was clear that his apetite was not satiated Britain and France made it known that they would not permit futher German agression to go unchallenged. Specifically they pledged that in the event of a German attack on Poland, they would declare war. That was fine with Hitler, war was what he wanted. Thus, when Poland's turn came, France and Britain gave Germany 24 hours to cease its attack and withdraw. It didn't, so they declared war.Michael Montagne
Directly after Munich Chamberlain sincerely beleived that Hitlers forceful take overs of Germanys surrounding countries would seize, by mid 1939 it was obvious that this was not going to happen. With England and France's promise to Poland they had to react... England gave Hitler till eleven on the morning of the 3rd of September 1939 to call his troops out of Poland, at 11 15 am Chamberlain adressed his country that this had not happened therefore England declared war on Germany. Because of Englands and Frances promise to Poland and to their countries (to ensure their safety from the dictatorships) that they must go to war... Other European countries than followed in surpport of either the allies or Germany.