a renewed series of seaborne assaults, which the imperial fleets proved unable to stem. In the West, Picts and Irish ships raided Britain, while the Saxons raided the North Sea, forcing the Romans to abandon Frisia. In the East, the Goths and other tribes from modern Ukraine raided in great numbers over the Black Sea. These invasions began during the rule of Trebonianus Gallus, when for the first time Germanic tribes built up their own powerful fleet in the Black Sea. Via two surprise attacks (256) on Roman naval bases in the Caucasus and near the Danube, numerous ships fell into the hands of the Germans, whereupon the raids were extended as far as the Aegean Sea; Byzantium, Athens, Sparta and other towns were plundered and the responsible provincial fleets were heavily debilitated. It was not until the attackers made a tactical error, that their onrush could be stopped. In 267–270 another, much fiercer series of attacks took place. A fleet composed of Heruli and other tribes raided the coasts of Thrace and the Pontus. Defeated off Byzantium by general Venerianus, the barbarians fled into the Aegean, and ravaged many islands and coastal cities, including Athens and Corinth. As they retreated northwards over land, they were defeated by Emperor Gallienus at Nestos. However, this was merely the prelude to an even larger invasion that was launched in 268/269: several tribes banded together (the Historia Augusta mentions Scythians, Greuthungi, Tervingi, Gepids, Peucini, Celts and Heruli) and allegedly 2,000 ships and 325,000 men strong, raided the Thracian shore, attacked Byzantium and continued raiding the Aegean as far as Crete, while the main force approached Thessalonica. Emperor Claudius II however was able to defeat them at the Battle of Naissus, ending the Gothic threat for the time being. Barbarian raids also increased along the Rhine frontier and in the North Sea. Eutropius mentions that during the 280s, the sea along the coasts of the provinces of Belgica and Armorica was "infested with Franks and Saxons". To counter them, Maximian appointed Carausius as commander of the British Fleet. However, Carausius rose up in late 286 and seceded from the Empire with Britannia and parts of the northern Gallic coast. With a single blow Roman control of the channel and the North Sea was lost, and emperor Maximinus was forced to create a completely new Northern Fleet, but in lack of training it was almost