World War 2 started with the invasion of Poland. The invasion was from the German military in which was led by Adolf Hitler. Adolf claimed, "It was a defensive action." but he had ulterior motives and Britain and France knew it too.
This was an important part of world war 2 because without the invasion then world war 2 might not have even happened. With Adolf invading Poland. It all happened because Adolf wanted to gain back lost territory and rule Poland again.