The Battle of Britannia was a battle fought on Magna Roma between Magna Roman soldiers and German tribesmen.
The Germans were lured to Britannia by rumors that the Romans had withdrawn most of their soldiers from the island. The German tribes then united temporarily and sent a force by sea, which was transported by the Norse, to the island. Upon landing, the Germans marched quickly on Londinium and were surrounded and destroyed by the Magna Romans.
In 2364, a holodeck simulation of this battle served as a training exercise for the crew of the USS Centurion. (TNG novel: The Captains' Honor)