In the Late 3rd Century environmental conditions around the Rhine, which gave rise to flooding and easier access to the sea allowed the Saxon and Frankish tribes to develop sea borne travel. It was at this time that the Roman Military started to abandon the are due to flooding. Which allowed the Saxon tribes more freedom to move and carry out raids.
With access to the open seas the Franks and Saxons began raiding into Roman Territories, in Britain the Fens became vulnerable to attacks, due to the coastline being easy to reach and in parts reached 30 miles inland compared to the modern coastline. As a result Estuaries and ports suffered problems, as these became easy targets for raiders.
The Saxons and Franks became raiders and pirates with the first recorded incidents between 260-278AD. The first major raid was in 260 AD in the aftermath of the capture of Valeria by the Persians. The Franks took the opportunity to raid Gaul, Sweeping through the country causing a lot of damage. The response by the central Roman Government was poor and this allowed a Roman army general called Postumus the ability to seize control of Britain, Gaul and Spain, whereby he created the Gallic Empire which lasted for 14 years.
During this period the Gothic tribes were also active in the Eastern half of the empire raiding over land and by sea around the Black and Aegean sea, where they sacked major cities such as Athens and Nicaea.
By the end of the late 3rd century there were raiders active right across the Roman Empire, from Scandinavia, Germania, Northern Britain in the form of the Picts, the Gothic, Huns and Alans in the East.