Boris I of Bulgaria
Louis the German was also planning to launch a major campaign against Rastislav with the support of Boris I of Bulgaria. Although at the last minute the latter pulled out of the campaign, Louis' new expedition against Rastislav was a success. In August 864 Louis the German invaded Moravia, crossing the Danube to besiege the "civitas Dowina" (identified, although not unanimously, with Devín Castle in Slovakia). The king apparently took Rastislav by surprise, and trapped him within the fortress. Unable to escape the Frankish siege, Rastislav surrendered, turned over a numerous high-ranking hostages and swore a new oath of fidelity.