The Battle of Kupa occurred at

Kupa The Kupa () or Kolpa ( or ; from la, Colapis in Roman times; hu, Kulpa) river, a right tributary of the Sava, forms a natural border between north-west Croatia and southeast Slovenia. It is long, with its border part having a length of and th ...

river in 819. It involved Frankish vassal Duke Borna of

Dalmatia Dalmatia (; hr, Dalmacija ; it, Dalmazia; see #Name, names in other languages) is one of the four historical region, historical regions of Croatia, alongside Croatia proper, Slavonia, and Istria. Dalmatia is a narrow belt of the east shore of ...

, with an army of

Guduscani The Guduscani or Goduscani ( hr, Guduščani, Gačani) were a tribe whose location and origin on the territory of early medieval Croatia remains a matter of dispute. According to one hypothesis they were located around present-day Gacka ( Lika), b ...

, against the advancing army of Frankish rebel, Duke

Ljudevit Ljudevit () is a Croatian masculine given name. The name comes from the word ''ljudi'', meaning ''people''. The name Ljudevit is also used as a translation of foreign names such as Ludwig or Louis. Ljudevit may refer to: * Ljudevit (Lower Pannon ...

of the

Slavs in Lower Pannonia Early Slavs settled in the eastern and southern parts of the former Roman province of Pannonia. The term ''Lower Pannonia'' ( la, Pannonia inferior, hu, Alsó-pannoniai grófság, sh-Latn-Cyrl, Donja Panonija, Доња Панонија, sl, Spo ...

. During the battle, the Guduscans abandoned Borna and joined Ljudevit. While Borna's forces suffered massive losses, he managed to escape with his bodyguards. However, Dragomuž, Ljudevit's father-in-law, who sided with Borna, was killed. Ljudevit suffered heavy casualties that included 3,000 soldiers and over 300 horses. Afterwards, Ljudevit used the momentum to invade Dalmatia in December 819.

Sources

9th-century military history of Croatia

819 9th century in Croatia {{Croatia-hist-stub