In 805 Doge Oberlerio degli Antinori--fearing a coup such as the one he had himself engineered against his predecessor--rashly committed Venice to the sovereignty of Charlemagne's Frankish empire (by then grandiloquently called the Holy Roman Empire). Charlemagne was seeking to consolidate his power in Northern Italy by nibbling away at areas that were--like Venice--historically subject to the sovereignty of the Eastern (or Byzantine) Roman Empire then centered at Constantinople [present-day Istanbul]. In 809, his political position having become even more tenuous, the Doge invited Charlemagne's son Pepin, whom Pope Hadrian had crowned as King of Italy, to send an armed force up the Adriatic coast from Ravenna to occupy Venice and its lagoon.

Shunting aside the hapless Doge, the people of the lagoon forgot all political differences and immediately formed a common defense under the leadership of Agnello Participazio (whose family in later generations assumed the surname Badoer). At the southern end of the Venetian lagoon, Chioggia and Palestrina fell quickly to Pepin's advance in the Spring of 810, together with Grado and perhaps Jesolo at the lagoon's Northern end. Then, however, Pepin was stymied by Venice's watery defenses. The Venetians removed all buoys and channel markers, making the shallow lagoon a dangerous maze of shoals and currents, impenetrable to Pepin's naval forces. The channel between Palestrina and the heavily-settled barrier island of Malamocco became an impassable obstacle and the islands of Rivo Alto [Ri'Alto] were even further removed from danger.

Pepin's forces spent six frustrating months encamped on the mainland, subject to debilating summer fevers, harassing attacks from the Venetians, and rumors of a Byzantine fleet sailing to support the Venetians. Finally, Pepin's weakened army withdrew, its pride salved by Venice's agreeing to pay an annual tribute. Pepin himself died within a few weeks thereafter, and his father Charlemagne abandoned his Venetian ambitions.

Greater than the military victory itself was the campaign's importance in forging among the lagoon dwellers a sense of unity, of community, that persisted for the next 1,000 years. The siege can be viewed as the event that defined Venice.