This event repeats every year forever
On this date German forces invaded Poland. It is generally the date that people used for the actual military conflict. Over the next couple of years various countries made formal declarations and entered the war, most notably would be the Japanese, but many other nations became engaged in the conflict.
The Axis nations were Germany, Italy and Japan
The Allied nations of France, England, Russia and the United States had many additional nations join in the cause or fight alongside these nations even though their own country had been overrun earlier in the war.