In the meantime, the Dalmatian Croats were recorded to have been subject to the Kingdom of Italy under Lothair I, since 828. The Croatian Prince Mislav (835–845) built up a formidable navy, and in 839 signed a peace treaty with Pietro Tradonico, doge of Venice. The Venetians soon proceeded to battle with the independent Slavic pirates of the Pagania region, but failed to defeat them. The Bulgarian king Boris I (called by the Byzantine Empire Archont of Bulgaria after he made Christianity the official religion of Bulgaria) also waged a lengthy war against the Dalmatian Croats, trying to expand his state to the Adriatic.