To understand what was at stake at Patras in the early ninth century, it helps to know what had happened to Greece in the seventh. When the Byzantine Empire was rocked by wars against Persia and then the early Arab caliphate, its frontier on the Danube collapsed and Slavic groups flooded southward — raiding as far as the Aegean coasts, settling in the Balkan interior. Most of Greece's inland cities were sacked or depopulated. Of the major centers, only coastal strongholds like Thessalonica remained firmly in imperial hands. The Peloponnese's western half came under Slavic occupation for roughly two centuries, during which the Slavic groups established their own local rulers while maintaining uneasy relations with Byzantine-held coastal towns. By the reign of Emperor Nikephoros I (802–811), Byzantine authority was being reasserted across Greece — but the process was unfinished, and the Peloponnesian Slavs were not reconciled to it.

The account preserved in Chapter 49 of Emperor Constantine VII's tenth-century work De administrando imperio describes the siege in careful detail. The city of Patras was holding out, but the inhabitants had begun to think of surrendering. Their envoy reached Corinth and learned that the military governor — the strategos — was delayed or not coming. Constantine VII notes dryly that the strategos arrived three days after the siege had ended. On the return journey, the rider's horse slipped. Both rider and flag fell. The inhabitants watching from the walls interpreted the fallen flag as a signal that help was on the way. They did not wait. They sallied out from the city gates against the besieging Slavic forces — and by tradition, as recorded in Constantine VII's account, their charge was led by the city's patron saint, Andrew the Apostle, appearing on horseback. The Slavs broke and fled, abandoning the siege entirely.

The date remains genuinely uncertain. Constantine VII gives no precise year, and scholars have debated whether the siege belongs to around 805 — when Patras was, according to the Chronicle of Monemvasia, effectively re-founded after a long period of disruption — or to 807, when an Arab fleet is independently documented to have reached southern Greece. The two accounts, Constantine VII's De administrando imperio and the Chronicle of Monemvasia (itself a work of contested accuracy), do not fully align. The Chronicle does not mention a siege at all; it records instead that an Armenian strategos named Skleros defeated the Peloponnesian Slavs around 804–806, and that Emperor Nikephoros I then undertook a major resettlement program, bringing descendants of Patras's original inhabitants back from Rhegion in Calabria — a program also confirmed by the chronicler Theophanes the Confessor, who dates it to around 810–811. Some scholars reconcile the two accounts by placing Skleros's military campaign first (c. 805) and the Slavic siege of Patras as a subsequent reaction (between 807 and 811).

Whatever its precise date, the repulsion of the siege had immediate consequences for the church as much as for the city. Patras had long been a suffragan see — subordinate to the Metropolis of Corinth. The city's deliverance, attributed to Saint Andrew's miraculous intervention, provided both spiritual and political grounds for elevation. Patras was raised to the status of a separate metropolis, acquiring its own ecclesiastical authority over other sees in the Peloponnese and rivaling its former superior in Corinth. The metropolitan of Patras gained significant political and financial influence that would shape the peninsula's church structures for centuries. For Byzantium more broadly, the successful defense of Patras secured the empire's maritime communications with Italy and the West — the shorter route through the Corinthian Gulf, rather than the longer and more exposed route around the southern Peloponnese. A stumbling horse had changed the balance of an empire.