After the successful war against Rascia, a medieval Serbian state, Bulgaria's ongoing expansion to the west reached Croatian borders. Bulgarian forces invaded Croatia approximately in 853 or 854 in northeastern Bosnia, where Croatia and Bulgaria bordered at the time.

According to available sources, there was only one major battle between the Bulgarian army and the Croatian forces. Sources say that the invading army led by the powerful Bulgarian Khan Boris I fought Duke Trpimir's forces on the mountainous territory of present-day northeastern Bosnia and Herzegovina in 854. The exact place and time of the battle is not known due to the lack of contemporary accounts of the battle. Neither Bulgarian nor Croatian side emerged victorious. Very soon afterward, both Boris of Bulgaria and Trpimir of Croatia turned to diplomacy and reached a peace treaty. Negotiations resulted in a long term establishment of peace with the border between the Duchy of Croatia and the Bulgarian Khanate stabilized at the Drina River (between modern-day Bosnia and Herzegovina and the Republic of Serbia).