What happened on september 1, 1939 that has to do with Germany's invades Poland. World War II begins.

Germany 'officially' invaded Poland in order to reclaim Danzig.

Explanation:

Germany, prior to World War Two, attempted to gain land for Germany and reunite ethnic Germans with their country. Such land gains included the annexation of the Sudetenland in Czechoslovakia, and the annexation of the city of Memel, among others. Germany sent Poland an ultimatum in the same fashion, asking them to surrender control of the city of Danzig, which had ethnic German roots. Poland did not comply, and Germany decided to invade. The invasion and conquest of Poland was most likely just for the sake of gaining land, and gaining a border with the USSR (which Germany later invaded).