This was an early cause of the war. In short in Europe there was an increase in dictators that used force to control their nation.
The Great Depression was right before the war (started in 1929 and ended in 1939). Tons of people became jobless, which means the economy was collapsing as well. People were becoming frustrated even World leaders.
Italy wasn't pleased with what they were given from the Treaty of Versailles. In return the join forces with Germany, the Axis Power.
Invasion on Poland
The Axis Power had three key power; Germany (Hitler), Italy (Mussolini), And Japan (Hirohito). During the war they had many areas under their power; like Norway, France, and Hungary.
The Allied Power consisted of three main countries; USA (Roosevelt), Great Britain (Churchill), and Soviet Union (Stalin).
September 1, 1939 marks the start of the war because Germany invaded Poland using their new Blitzkrieg tactics. Blitzkrieg is a kind of war emphasizing rapid and mechanized movement.