in related documents) and Michael of Zahumlje. According to some historians, Michael recognized Tomislav's rule (making Zachlumia a vassal state of Croatia). Tomislav did not protest the council's decision. Bishop Gregory appealed to the pope, and a second council (also in Split) was convened in 928 to resolve the controversy and enforce the first council's conclusions. The supremacy of the metropolitan archbishopric of Split was confirmed, and the Diocese of Nin was abolished. War with Bulgaria During Tomislav's rule, the Bulgarian and Byzantine Empires were at war. The Bulgarians under Emperor Simeon I destroyed the Principality of Serbia (a Byzantine ally) in 924, forcing Serbian Prince Zaharija and part of the Serbian population to flee to Croatia. Croatia (also a Byzantine ally) was now located between Bulgaria and the weakly-defended Byzantine Theme of Dalmatia. Tomislav may have received some control of the Theme of Dalmatia's coastal cities or a share of collected taxes for his assistance to the Byzantine Empire. Although Byzantium gave Tomislav the honorary title of proconsul, there is no evidence that it recognized the loss of its rights in the Theme of Dalmatia to Tomislav. Since Croatia was harboring Bulgarian enemies and was allied with the Byzantine Empire, Simeon attacked with an army led by Duke Alogobotur. Tomislav cut off Alogobotur's advance into Croatia and destroyed his army at the Croatian–Bulgarian battle of 926, which probably took place in eastern part Bosnia. After Simeon's death in 927, Pope John X sent legates with Bishop Madalbert to mediate between Croatia and Bulgaria and restored peace. It is unknown how Tomislav died, but he disappeared from the political scene after 928 and was succeeded by Trpimir II. Geographic extent The geographic extent of Tomislav's kingdom is not fully known. John the Deacon, whose chronicle is a primary source of the history of the 9th-and 10th-century Slavic peoples in Dalmatia, wrote that in 912 a Venetian ambassador returning from Bulgaria passed through Croatian territory before reaching the land of Zahumlje under Duke Michael; this suggests that Tomislav's Croatia bordered Bulgaria, then under the rule of Simeon I. British writer Marcus Tanner suggested that it covered most of modern Croatia, Bosnia and Herzegovina, and the coastline of Montenegro. According to Roger Lampe, the state did not extend as far south as Dubrovnik and Istria was not included. Many Croatian scholars said that the kingdom covered the region south of the Drava