Bulgaria signs the Tripartite Pact, officially becoming a member of the Axis Powers. The original three members of the are Nazi Germany, Fascist Italy, and Imperial Japan. At this time, Hungary, Romania, and Slovakia had also joined.
Bulgaria's motivation for joining Nazi Germany seems to be a desire to reclaim land lost in the aftermath of World War I. If they hadn't signed the treaty and joined the Axis Powers, though, they would have been invaded: Nazi Germany troops were right on Bulgaria's border, preparing to invade Yugoslavia and Greece.