The 7th-century Frankish "Chronicle of Fredegar" wrote that the Avars employed the Slavs, or Wends, as "Befulci", showing that the Slavs formed special military units in the Avar Khaganate. According to the same chronicle, the Wends rose up in rebellion against their Avar masters and elected a Frankish merchant, Samo, their king "in the fortieth year of Clothar's reign", that is in 623 or 624. Modern historians agree that the Avars' defeat during the siege of Constantinople in 626 enabled Samo to consolidate his rule. He routed the invading army of Dagobert I, King of the Franks, in the Battle of Wogastisburg in 631 or 632. The realm of Samo, who ruled for 35 years, collapsed soon after his death. Its exact borders cannot be determined, but it must have been located near the confluence of the Danube and the Morava rivers. Historian Richard Marsina puts its centre to Lower Austria.