That depends on which country. Germany, France, Poland and Britain entered the war when Germany invaded Poland. Germany at the time included Austria and Czechoslovakia. Belgium, the Balkans and Russia entered upon German invasion at different times. Italy came in during Germany's assault on France. Japan and the United States both joined the war after Pearl Harbor
The official beginning of the entire conflict was the invasion of Poland.
France
September 1939 is the month the historians use as the beginning of World War 2.
September 1, 1939 was the official beginning of World War 2.
France
1939
FDR
Neutral
by radio
the Jews
Sep 1 1939
they wern't
World War 2