Simeon I planned to secure his position in Constantinople through a marriage between his daughter and the infant Emperor Constantine VII (r. 913–959), thus becoming basileopator (father-in-law) and guardian of Constantine VII. However, in 919 Admiral Romanos Lekapenos married his daughter to Constantine VII and in 920 proclaimed himself senior emperor, ruining Simeon I's ambitions to ascend the throne by diplomatic means. Until his death, the Bulgarian monarch never recognized the legitimacy of Romanos' accession to the throne. Thus, in the beginning of 921 Simeon I did not reply to a proposal of the Ecumenical Patriarch Nicholas Mystikos to betroth one of his daughters or sons to a progeny of Romanos I and sent his army into Byzantine Thrace, reaching Katasyrtai in the outskirts of Constantinople.

The Battle of Pegae took place in a locality called Pegae (i.e. "the spring"), named after the nearby Church of St. Mary of the Spring. The Byzantine lines collapsed at the very first Bulgarian attack and their commanders fled the battlefield. In the subsequent rout most Byzantine soldiers were killed by the sword, drowned or were captured.

In 922 the Bulgarians continued their successful campaigns in Byzantine Thrace, capturing a number of towns and fortresses, including Adrianople, Thrace's most important city, and Bizye. In June 922 they engaged and defeated yet another Byzantine army at Constantinople, confirming the Bulgarian domination of the Balkans. However, Constantinople itself remained outside their reach, because Bulgaria lacked the naval power to launch a successful siege. The attempts of the Bulgarian emperor Simeon I to negotiate a joint Bulgarian–Arab assault on the city with the Fatimids were uncovered by the Byzantine and countered.