Simeon I of Bulgaria was not an ordinary conqueror. Raised in Constantinople and educated at the Magnaura School, he spoke Greek, read the church fathers, and understood Byzantine court politics from the inside. He had ruled Bulgaria since 893 and was pursuing something extraordinary: the imperial throne of Constantinople itself. His first approach was diplomatic—he sought to marry his daughter to the young emperor Constantine VII and thereby become basileopator, father-in-law and effective regent of the empire. That plan collapsed when Admiral Romanos Lekapenos outmaneuvered him, marrying his own daughter to Constantine VII in 919 and proclaiming himself senior emperor the following year. Simeon never accepted the legitimacy of that move. What diplomacy had failed to deliver, war would have to win.

The road to Pegae ran through four years of relentless Bulgarian campaigning. After the catastrophic Byzantine defeat at the Battle of Achelous in 917 and the follow-up rout at Katasyrtai the same autumn, Simeon's armies had ranged through Byzantine Thrace nearly unchallenged, reaching the walls of Constantinople and raiding as far south as the Isthmus of Corinth. In 921, the Bulgarian tsar sent a large force under commanders named Kaukanos and Menikos toward the capital while he himself led another army to besiege Adrianople, the most important city in Thrace. Emperor Romanos I responded by dispatching the Domestic of the Schools Pothos Argyros, his brother Leo Argyros, the admiral Alexios Mosele, and an official called John the Rhaiktor with the tagmata—the professional units of the imperial army—and the Hetaireia, the imperial guard itself.

The Bulgarian army that arrived at Pegae between March 11 and 18 did not negotiate. They charged with what the Byzantine sources call a dreadful battle cry, and their first strike broke the imperial lines. John the Rhaiktor fled immediately. Many of the soldiers who died at Pegae died not fighting the Bulgarians but covering their commander's retreat—killed protecting a man who had already decided to run. In the rout that followed, most of the Byzantine soldiers were cut down by sword, drowned as they tried to escape across water, or taken prisoner. It was a swift and total collapse. The men of the tagmata and the imperial guard—trained professionals who were supposed to be the finest soldiers in the empire—were destroyed in the fields beside a sacred spring, their deaths as swift and undignified as any in Byzantine history.

Pegae was not the end of Simeon's campaign; it was barely a pause. In 922 the Bulgarians captured Adrianople, which surrendered after a long siege. Another Bulgarian army engaged and defeated yet another Byzantine force near Constantinople. Simeon was attempting to arrange a joint assault on the city with the Fatimid Caliphate, a diplomatic maneuver that the Byzantines managed to uncover and counter—they intercepted the ship carrying Bulgarian and Fatimid envoys and outbid the alliance. Constantinople, protected by its triple walls and access to the sea, remained beyond Bulgarian reach as long as Simeon lacked a fleet capable of blockading it. He died in 927 without taking the city he had spent decades trying to claim. The spring at Pegae still rose from the earth, the church above it still stood. The soldiers who drowned and bled in those fields in March 921 were buried where they fell, in ground that has been Istanbul for more than five hundred years.