They were creating an alliance despite the fact that Communists (Stalin) hate Fascists (Hitler) and vice verse. They set aside their differences in order to come together since they were both powerful dictators that controlled menacing nations.
They banded together publicly under the Axis Powers, but they also secretly planned to split Poland between them. They (Stalin and Hitler/Russia and Germany) both promised and agreed to not attack the other in a treaty...but Hitler broke the treaty and attacked Stalin and the USSR...(This is known as one of Hitler’s greatest mistakes.)
This is what caused Stalin and the USSR to join the Allied Powers. (Great Britain, France and later, USA.)
This left Hitler’s Germany to be fighting only beside Mussolini’s Italy. They also had an alliance with Japan, but since Japan was so far away, they weren’t involved in the World Wars much and focused solely on conflicts outside of Europe.
