568: Wibbandun
The first battle Ceawlin fought as king is dated by the Chronicle to 568, when he and Cutha fought with Æthelberht, the king of Kent. The entry says "Here Ceawlin and Cutha fought against Aethelberht and drove him into Kent; and they killed two ealdormen, Oslaf and Cnebba, on Wibbandun." The location of "Wibbandun", which can be translated as "Wibba's Mount", has not been identified definitely; it was at one time thought to be Wimbledon, but this now is known to be incorrect. This battle is notable as the first recorded conflict between the invading peoples: previous battles recorded in the Chronicle are between the Anglo-Saxons and the native Britons.
There are multiple examples of joint kingship in Anglo-Saxon history, and this may be another: it is not clear what Cutha's relationship to Ceawlin is, but it certainly is possible he was also a king. The annal for 577, below, is another possible example.
Read more about this topic: Ceawlin Of Wessex