A third, even fourth rank navy to standards of the time, the very modest Bulgarian Navy has been curtailed and limited by the peace treaty of Neuilly in 1920 which decided the fate of Balkan states. In the interwar, the Bulgarian government started a rapprochement with Germany and Italy, which led to a reinforcement of the fleet, with the four main vintage Torpedo Boats being rearmed and modernized, and four new German Torpedo boats being purchased, as well as several misc. ships and patrol boats.
There has been an attempt to obtain the delivery of German submarines, but it failed as the war started. Bulgaria played a small role to secure the southern flank of the Axis in the Black sea, participating in Operation Varna, several minefields that would claim four Soviet submarines, and securing its main lines of communications. However, Bulgaria swapped side when the Soviet Army arrived in the sumer of 1944, the whole fleet being captured at Varna.
Read More about the Bulgarian Fleet during WW2