Why did Bulgaria support the Axis powers (Germany, Italy, Japan) from 1941-44?
Ever since the C19, Bulgarian nationalists regarded Macedonia as rightfully Bulgarian and in both WWI and WWII thought it would be easy to make good their claim. (In WWII they also wanted to regain territory lost to Greece at the end of WWI). In both wars the Bulgarian government thought it could fight a local war “on the back” of a major war between European powers. See also the link provided by gmack on question #57914.