On August 23, 1939, Hitler and Stalin signed the German-Soviet Nonaggression Pact.
The name of the pact is quite obvious for what it's for. The Soviet Union (Stalin) and Germany (Hitler) signed a nonaggression pact. Hitler only wanted to sign the pact with the Soviet Union to invade Poland (which triggered the beginning of WWII). The pact stated most of the simple things: no attacking each other, no joining groups of powers that threaten either one of the two parties, etc.