What was the Tripartite Pact as noted in the biography, Adolf Hitler?

The Tripartite Pact was a treaty between Germany, Italy and Japan for cooperation. It was nominally aimed at the Soviet Union, but Foreign Minister Ribbentrop got the idea of having the Soviet Union join the Pact in 1940. This was discussed with Soviet representatives but never actually happened. Instead, Hitler attacked the Soviet Union in June of 1941.