Bulgaria participated in the Axis invasion of Yugoslavia and Greece by letting German troops attack from its territory and sent troops to Greece on April 20. As a reward, the Axis powers allowed Bulgaria to occupy parts of both countries—southern and south-eastern Yugoslavia (Vardar Banovina) and north-eastern Greece (parts of Greek Macedonia and Greek Thrace). The Bulgarian forces in these areas spent the following years fighting various nationalist groups and resistance movements. Despite German pressure, Bulgaria did not take part in the Axis invasion of the Soviet Union and actually never declared war on the Soviet Union. The Bulgarian Navy was nonetheless involved in a number of skirmishes with the Soviet Black Sea Fleet, which attacked Bulgarian shipping.