The Croatian–Bulgarian Wars were a series of conflicts that erupted three times during the 9th and 10th centuries between the medieval states of Bulgaria and Croatia. During these wars, the Croatian state was a vassal state or formed alliances with Eastern Francia and Byzantium against the Bulgarian Empire. First battle During the middle of the 9th century, Bulgaria was the dominant power in the central, eastern, and northern Balkans. In 854, the Bulgarian ruler Boris I forged an official alliance with the Moravian prince Rastislav against Louis the German of East Francia. Duke Trpimir of Croatia was a faithful Frankish vassal and was wary of Bulgaria's ongoing expansion when it reached Croatian borders after the wars against Serbia. Bulgaria is said to have invaded Croatia in approximately 854, but there is also a possibility that King Louis gave some compensation to Trpimir to attack Bulgaria. During the war of 854, there was only one great battle on the territory of present-day northeastern Bosnia, and neither side emerged victorious. Soon afterward, peace negotiations began between Boris of Bulgaria and Trpimir of Croatia, resulting in gifts exchanged and the establishment of peace, with the border between the Croatian Duchy and Bulgaria stabilized at the river of Drina (between modern-day Bosnia and Herzegovina and Republic of Serbia). Second war A very long and arduous war was prosecuted between the Bulgarian Tsar Simeon I, who wanted to take over the Byzantine Empire, and the Empire's monarch Romanos I. Under heavy pressure from the Bulgarians and amassing defeat after defeat, the Byzantine Empire negotiated with Serbia and Croatia in an effort to forge an alliance against the Bulgarians. Having been informed of these plans by Prince Michael of Zachlumia, who was forced to the islands as the Serbian rulers took direct control of most of his local lands for themselves, Simeon overran Serbia in 924, destroyed it by tricking its nobility, and made it directly part of the Bulgarian Empire. Serbia's ruler Zaharija Pribislavljević fled and found exile at the Croatian court, while after the realm's destruction massive waves of Serb refugees fled and found refuge in Tomislav's Croatia. Trying to gain Croatian entry into this coalition, Romanos I ordered the province of Dalmatia in ca. 925 to pay taxes not to Byzantium but rather to the Croatian state; the Dalmatian territories, including most cities and the northern islands, were henceforth administered by King Tomislav