Simeon's successful campaigns against the Byzantines, Magyars and Serbs led Bulgaria to its greatest territorial expansion ever,[3] making it the most powerful state in contemporary Eastern and Southeast Europe.

Vladimir attempted to reintroduce paganism in the empire and possibly signed an anti-Byzantine pact with Arnulf of Carinthia,[22] compelling Boris to re-enter political life.

[35] In addition, the talented commander Nikephoros Phokas was called back from southern Italy to lead a separate army against Bulgaria in 895 with the intention to merely overawe the Bulgarians.

Simeon did not trust the envoy and, after sending him to prison, ordered the Byzantine navy's route into the Danube closed off with ropes and chains, intending to hold it until he had dealt with Phokas.

[38] Despite the problems they encountered because of the fencing, the Byzantines ultimately managed to ferry the Magyar forces led by Árpád's son Liüntika across the Danube,[39] possibly near modern Galaţi,[40] and assisted them in pillaging the nearby Bulgarian lands.

Once notified of the surprise invasion, Simeon headed north to stop the Magyars, leaving some of his troops at the southern border to prevent an attack by Phokas.

[16][50] The war ended with a peace treaty which formally lasted until around Leo VI's death in 912[7] and under which Byzantium was obliged to pay Bulgaria an annual tribute.

In order to dissuade Simeon from capturing the city and populating it with Slavs,[16][56] Leo VI was forced to make further territorial concessions to the Bulgarians in the modern region of Macedonia.

[60] Many residents of Constantinople did not recognize the young emperor and instead supported the pretender Constantine Doukas,[61] which, exacerbated by revolts in southern Italy and the planned Arab invasion in eastern Anatolia, was all to Simeon's advantage.

[62] Nicholas Mystikos tried to discourage Simeon from invading Byzantium in a long series of pleading letters, but the Bulgarian ruler nevertheless attacked in full force in late July or August 913, reaching Constantinople without any serious resistance.

[66][67] 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.

Zoe was quick to send Simeon numerous presents in order to conciliate him, and she managed to convince him to cede back Adrianople and withdraw his army.

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).

In the Battle of Acheloos of 20 August 917, one of the largest in medieval history,[76] the Bulgarians completely routed the Byzantines and killed many of their commanders, although Phokas managed to escape to Mesembria.

Romanos betrothed his daughter Helena Lekapene to Constantine and advanced to the rank of co-emperor in December 920, effectively assuming the government of the empire,[83][84] which was largely what Simeon had planned to do.

[16] Simeon's forces appeared before Constantinople in 921, when they demanded the deposition of Romanos and captured Adrianople, and 922, when they were victorious at Pigae, burned much of the Golden Horn and seized Bizye.

[89] The Bulgarian emperor sent his troops under Sigrica and Marmais, but they were routed and the two commanders beheaded, which forced Simeon to conclude an armistice with Byzantium in order to concentrate on the suppression of the uprising.

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.

[95] In 926, Simeon's troops under Alogobotur invaded Croatia, at the time a Byzantine ally, but were completely defeated by the army of King Tomislav in the Battle of the Bosnian Highlands.

[7] Fearing a Bulgarian retribution, Tomislav agreed to abandon his union with Byzantium and make peace on the basis of the status quo, negotiated by the papal legate Madalbert.

[102] H.H.Howorth opined "If he had lived, or if he had been succeeded by princes of the same martial character, it is very probable that a great Slav state reaching from the Adriatic to the Black Sea, which would have been a barrier to the Turks, might have been formed south of the Danube.

[2][104] In this respect, Simeon continued his father Boris' policy of establishing and spreading Slavic culture and attracting noted scholars and writers within Bulgaria's borders.

[105] Having spent his early years in Constantinople, Simeon introduced Byzantine culture to the Bulgarian court, but eliminated its assimilative effect by means of military power and religious autonomy.

[105] The disciples of Cyril and Methodius, among whom were Clement of Ohrid, Naum and Constantine of Preslav, continued their educational work in Bulgaria, actively translating Christian texts, such as the Bible and the works of John Chrysostom, Basil of Caesarea, Cyril of Alexandria, Gregory of Nazianzus, and Athanasius of Alexandria, as well as historic chronicles such as those of John Malalas and George Hamartolus, to Bulgarian.

[105] The reign of Simeon also witnessed the production of a number of original theological and secular works, such as John Exarch's Six Days (Šestodnev), Constantine of Preslav's Alphabetical Prayer and Proclamation of the Holy Gospels, and Černorizec Hrabǎr's An Account of Letters.

[106] Simeon turned the new Bulgarian capital Preslav into a magnificent religious and cultural centre, intended more as a display of his realm's heyday and as a royal residence than as a military fortress.

Bulgarian national writer Ivan Vazov dedicated a children's patriotic poem to him, "Tsar Simeon", and it was later arranged as a song, "Kray Bosfora šum se vdiga" ("A Clamour Rises by the Bosphorus").

[115] A historical drama play called Tsar Simeon Veliki – Zlatniyat vek produced by Stefan Staychev, director of the Silistra Theatre, premiered in December 2006.