Campaign against the German Kingdom

In 853, relations between Boris I and Louis the German, king of the German Kingdom, deteriorated sharply. Bulgaria, allied with the Moravians, launched an attack against the German Kingdom.

Historians suggest that the Slavs who joined the Bulgarian troops were subjects of Rastislav, prince of Great Moravia. With the Bulgarian ruler Boris's assistance, he sought to curb the German Kingdom's expansionist ambitions towards Great Moravia. The Bulgarian ruler himself probably sought expansion in the middle Danube region. The military venture, however, quickly began to turn into a fiasco.

Louis the German instigated war against the Bulgarians by manipulating the Croatian prince Trpimir, who was politically dependent on him, into attacking Bulgaria. This forced the Bulgarian forces to divide their efforts and fight on two fronts. According to the Byzantine emperor Constantine VIII, however, the troops Boris sent against Trpimir achieved no success.

The attack ended in a Bulgarian defeat. 

According to a chronicle note contained in the letters of Theophylact of Ohrid, “a cloud of Franks covered all Bulgaria”. This probably means that after the Bulgarian army's defeat in Croatia, Louis the German's troops penetrated deep into Bulgaria, forcing the Bulgarians to withdraw from the war and seek renewed rapprochement with the German Kingdom. In 855, no Bulgarian forces are recorded as supporting Prince Rastislav in his efforts to resist Louis the German's pressure on Great Moravian territory.

Thus, instead of strengthening Bulgarian political influence in the middle Danube region, the campaign of 853–854 left Bulgaria shaken by its military defeats.