Byzantine–Bulgarian wars
Simeon I the Great Imperial ambitions

With the ascendance of Simeon I to the throne in 893, the long-lasting peace with the Byzantine Empire established by his father was about to end. A conflict arose when Byzantine Emperor Leo VI the Wise, acting under pressure from his wife Zoe Zaoutzaina and her father, Stylianos Zaoutzes, moved the marketplace for Bulgarian goods from Constantinople to Thessaloniki, where Bulgarian merchants were heavily taxed. Forced to take action, in the autumn of 894 Simeon invaded the Byzantine Empire from the north, meeting little opposition due to the concentration of most Byzantine forces in eastern Anatolia to counter Arab invasions. Informed of the Bulgarian offensive, the surprised Leo sent an army consisting of guardsmen and other military units from the capital to halt Simeon, but his troops were routed somewhere in the theme of Macedonia.The Magyars managed to defeat Simeon's army twice, but in 896 they were routed in the Battle of Southern Buh. The war ended in 896 with a great Bulgarian victory near Bulgarophygon in Eastern Thrace. The market was returned to Constantinople, and the Byzantine Emperor had to pay annual tribute to Bulgaria. More importantly, with help from the Pechenegs Simeon successfully fended off a Magyar invasion, which was coordinated with the Byzantines.

After the death of Leo VI on 11 May 912 and the accession of his infant son Constantine VII, under the guidance of Leo's brother Alexander, who expelled Leo's last wife and Constantine's mother, Zoe Karbonopsina, from the palace, Simeon claimed the imperial title and tried to replace Byzantium as the biggest power in the region, perhaps building a new Bulgarian–Byzantine empire. Alexander died on 6 June 913, leaving the capital in anarchy and the rule of the empire in the hands of a regency council headed by Patriarch Nicholas Mystikos. This gave the Bulgarian ruler a great opportunity to attempt a campaign towards the Byzantine capital, so he attacked in full force in late July or August 913, reaching Constantinople without any serious resistance. The protracted negotiations resulted in the payment of the arrears in the Byzantine tribute, the promise that Constantine VII would marry one of Simeon's daughters, and, most importantly, Simeon's official recognition as Emperor (tzar) of the Bulgarians by Patriarch Nicholas in the Blachernai Palace. Until the end of his reign, Simeon used the style of "Emperor of the Bulgarians and the Romans".

Shortly after Simeon visited Constantinople, Constantine's mother Zoe returned to the palace on the insistence of the young emperor and immediately proceeded to eliminate the regents. Through a plot, she managed to assume power in February 914, practically removing Patriarch Nicholas from the government, disowning and obscuring his recognition of Simeon's imperial title, and rejecting the planned marriage of her son to one of Simeon's daughters. In retaliation, Simeon invaded Thrace in the summer of 914 and captured Adrianople. In 917, a particularly strong Byzantine army led by Leo Phokas, son of Nikephoros Phokas, invaded Bulgaria accompanied by the Byzantine navy under the command of Romanos Lekapenos, which sailed to the Bulgarian Black Sea ports. En route to Mesembria (Nesebǎr), where they were supposed to be reinforced by troops transported by the navy, Phokas' forces stopped to rest near the river of Acheloos, not far from the port of Anchialos (Pomorie). Once informed of the invasion, Simeon rushed to intercept the Byzantines, and he attacked them from the nearby hills while they were resting disorganized. In the Battle of Achelous (or Anchialus) on 20 August 917, one of the largest in medieval history, the Bulgarians completely routed the Byzantines and killed many of their commanders, although Phokas managed to escape to Mesembria. As a result of the victory Simeon drew into his orbit the Pecheneg leaders and started a major offensive against the European dominions of Byzantium. The Bulgarians sent to pursue the remnants of the Byzantine army approached Constantinople and encountered Byzantine forces under Leo Phokas, who had returned to the capital, at the village of Katasyrtai in the immediate proximity of Constantinople.

Simeon pursued an aggressive policy regarding the Medieval Serbian principalities that tended to support Byzantium. Bulgarian troops led by Theodore Sigritsa and Marmais invaded the country, deposing local rulers like Petar Gojniković and Pavle Branović. Meanwhile, the admiral Romanos Lekapenos replaced Zoe as regent of the young Constantine VII in 919 and advanced himself to the rank of co-emperor in December 920, effectively assuming control of the empire. No longer able to climb to the Byzantine throne by diplomatic means, the infuriated Simeon once again had to wage war to impose his will. Between 920 and 922, Bulgaria increased its pressure on Byzantium, campaigning in the west through Thessaly, reaching the Isthmus of Corinth, and in the east in Thrace, reaching and crossing the Dardanelles to lay siege on the town of Lampsacus. Simeon's forces appeared before Constantinople in 921, when they demanded the deposition of Romanos and captured Adrianople; in 922 they were victorious at Pigae, burning much of the Golden Horn and seizing Bizye.

Desperate to conquer Constantinople, Simeon planned a large campaign in 924 and sent envoys to the Shia Fatimid ruler Ubayd Allah al-Mahdi Billah, who possessed a powerful navy, which Simeon needed. The Ubayd Allah agreed and sent his own representatives back with the Bulgarians to arrange the alliance. However, the envoys were captured by the Byzantines at Calabria. Romanos offered peace to Egypt under the Fatimids, supplementing this offer with generous gifts, and ruined the Fatimids newly formed alliance with Bulgaria.

In 924 Simeon sent an army led by Časlav Klonimirović to depose a former ally of his, Zaharije Pribisavljević. He was successful, as Zaharije fled to Croatia. In the summer of the same year, Simeon arrived at Constantinople and demanded to see the patriarch and the emperor. He conversed with Romanos on the Golden Horn on 9 September 924 and arranged a truce, according to which Byzantium would pay Bulgaria an annual tax, but would be ceded back some cities on the Black Sea coast. In 926, Simeon's troops invaded Croatia, at the time a Byzantine ally, but were severely defeated by the army of King Tomislav in the Battle of the Bosnian Highlands. A peace was mediated by the papal legate Madalbert between Simeon and Tomislav. Though the army he sent to Croatia was destroyed, Simeon retained sufficient military forces to contemplate renewed aggression against the Byzantines.

After 14 years of war Simeon was ultimately too frustrated in his designs on the Byzantine throne. In the year following the destruction of his army in Croatia, while planning another attack on the Byzantines, he died of a heart attack in his palace in Preslav on May 27, 927.
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