Why Germany and Bulgaria become allies?

Bulgaria never became an ally of Germany during the Second World War, and instead the two countries were enemies. This is despite Bulgaria being a member of the Axis powers; as a result of German pressure and the hope that Bulgaria would be able to regain territory lost to the Allies in the First World War. German troops moved into Bulgaria in March 1941, which helped enable Bulgaria to attack Greece in return for control over Western Thrace. The Bulgarians continued to fight alongside the Germans on other fronts, but this never extended to becoming a firm ally.