World War II started on September 17, 1939, with the joint Soviet and Nazi invasion of Poland. Following this invasion, Britain and France declared war on Germany.
The Nazis, or National Socialist Party, steadily rose to power in Germany for a number of years before the war. Adolf Hitler became Reich Chancellor in 1933 and self-appointed Fuhrer in 1934.
When Hitler signed a non-aggression pact with Joseph Stalin in August 1939, British and French officials became worried about his intentions.
Following the invasion and defeat of Poland, Allied and German navies were locked in a conflict that resulted in the sinking of over 100 vessels before the end of 1939.