The Menapii was one of the Belgae tribes with a Celtic culture. They seem to belong to the same branch of Celts who migrated to the Atlantic coast and invaded Ireland. Their name might have been derived from a Proto-Celtic word -Mano meaning thought or -Mono meaning to tower. The Eburones were the Menapii's neighbours to the east, the Germanic Batavi to the North, the Nervii to their southeast and the Atrebates to their south. They obviously had no neighbours in the west because that is were the North sea is located.
The location of this tribe is interesting, Julius Caesar, Ptolemy and Strabo described that their territory was located between the rivers Rhine and the Schelde. This means that they should have lived in modern day Northern Belgium and Southern Netherlands. According to Caesar, the Menapii were the Celts that lived closed to the Rhine and thus they waged a continuously war against the Germanic people.
The Menapii were first mentioned by Julius Caesar in his work 'De Bello Gallico'. The Belgae tribes entered into a confederacy out of fear that the Romans would conquer them. The Menapii tribe was part of this confederation in 57BC.
This meant that the Menapii were at war with Julius Caesar and they were one of the very few tribes who managed to resist Caesar for quite some years.
In 56BC Julius Caesar left Illyricum and tensions flared up again almost immediately. Some of the Celtic tribes revolted against Caesar: the Veneti, Lexovii, Morini, Nannetes, Ambilati, Osismii and the Menapii. Especially the Veneti could prove dangerous to Rome since they had a powerful navy.
Unfortunately the Veneti fleet was cornered and destroyed in Quiberon Bay (modern day France). Most of the Veneti strongholds and villages were attacked and almost all of the Veneti civilians were killed or captured. The Menapii survive this slaughter and continued resisting Julius Caesar in what you can call guerilla warfare. They would carefully withdraw into their swamps and forests, luring Caesar's troops with them. This eventually led to the Romans burning down random villages.
To make matters even worse, the Germanic Suebi confederation decided to cross the Rhine in the following year and unfortunately the Menapii were in their way. This led to a clash between the Suebi and the Menapii. The Menapii are able to defend their line of the river. The Suebi decided to withdraw in an attempt to lure the Menapii out. This works and the Germanics were able to defeat the Menapii and occupy most of their villages and ships.
This event unfortunately drew Caesar's attention and in the year 55BC, Julius caesar attacked the Suebi and pushed them back across the Rhine again. By this time, most of the Belgae tribes submitted to Caesar but the Menapii and Morini refused. The Morini are eventually beaten in battle but the Menapii managed to avoid a slaughter by hiding inside a forest. The Belgae tribes became part of the Roman empire and they lost their independence forever.
Caesar's conquest of Gaul caused quite some panic amongst Celtic tribes. Some of these tribes decided to flee and there is reason to believe that parts of the Menapii tribe fled to Ireland. Ptolemy mentions a Manapi tribe living in south-east Ireland which could be remnants of the Menapii tribe.
Some of the Menapii apparently also served in the Roman army as auxiliaries. A cohort of Menapii auxiliaries is mentioned in inscriptions found in Britain dating back to somewhere around 200AD.
An even more interesting story about the Menapii revolves around Marcus Aurelius Mausaeus Carausius during the 2nd century AD. Carausius was a Menapii man in service of the Roman army. He managed to become the commander of the Classis Britannica, a Roman fleet active in what is now the English channel.
Carausius declared himself emperor of Britain and northern Gaul in 286AD. He managed to get the support of the legions stationed in Brittanica and Gaul and managed to stay in power for 7 years before being assassinated by Allectus, Carausius' treasurer. So it wasn't just only a Germanic Batavi who managed to become a Roman emperor (Postumus), the Celtic Menapii managed to produce an emperor as well.
The Menapii eventually got absorbed by the Franks and the Menapii who migrated to Ireland eventually became part of the kingdom of Laigin. This Celtic tribe definitely has a fascinating history, its future tied to that of the Germanics, Romans and the Irish.
Here are pictures of:
A map showing the territory of the Menapii,
A map of Ireland showing the location of the Manapii,
A coin with a depiction of Carausius,