After WWI, there was a period of time in between before WWII, and this time period, in a way, ignited the flame to begin WWII.
WWII was six years long, and it started when Germany did not follow the Treaty of Non-Aggression and invaded Poland in August 1939 under the Nazi leader, Adolf Hitler.
WWII ended after the United States bombed Japanese cities Hiroshima and Nagasaki, and Japan then surrendered on September 2nd, 1945.
World History Blog 