Why was Hadrian’s Wall built?

What the one ancient source says

A single ancient sentence explains the Wall. Hadrian’s biography in the Historia Augusta, written some two and a half centuries later, says he went to Britain, put many things right, and was the first to build a wall, eighty miles long, to divide the barbarians from the Romans (Hadrian 11.2). That is all. Everything else comes from the stones, the inscriptions and the forts themselves.

The background explains the rest. When Hadrian became emperor in 117, the same biography says, the Britons could not be kept under Roman control, while the Moors raided and Libya and Palestine stirred (Hadrian 5.2). His answer, across the empire, was to stop conquering. He handed back Trajan’s new lands beyond the Euphrates, Mesopotamia among them, because they could not be held. Where no river guarded the frontier, he closed it with palisades of great stakes (Hadrian 12.6). Britain got the grandest barrier of all.

Rome had already gone further

Forty years earlier Rome had tried to take the whole island. Tacitus’s father-in-law, Agricola, marched deep into Caledonia, today’s Scotland, and won a battle at Mons Graupius around AD 83. Before it, Tacitus gives the Caledonian chief Calgacus a speech against Rome:

To robbery, slaughter, plunder, they give the lying name of empire; they make a solitude and call it peace.

The words are Tacitus’s own, but the problem was real. The far north needed more soldiers than Rome would spare, and the legions pulled back. Tacitus later complained that Britain was conquered and at once let go (Histories 1.2). Hadrian drew his line where Rome’s grip actually held, across the narrow neck of land between the Tyne and the Solway.

How the Wall worked

Soldiers of three legions, perhaps 15,000 men, built most of it in about six years. The eastern stretch was stone; the western one was first built of turf and later rebuilt in stone. At every Roman mile stood a small fort with a gateway, a milecastle, and between each pair, two turrets. Sixteen larger forts were later added along the line, and behind it ran a broad ditch between earth banks, the Vallum.

Those gateways are the point. A wall with some eighty gates was not meant to seal Britain off. Most historians see it as a filter. It stopped raiding parties, slowed smuggling and made every traveler, trader and herd cross where soldiers could watch them, and most likely collect a fee. It also told everyone, on both sides, that Rome meant to stay.

Letters found at Vindolanda, a fort just behind the line, show how the garrison saw its neighbors, even before the Wall. Written around AD 100 on thin wooden tablets, one calls the locals Brittunculi, little Brits.

Not the border with Scotland

The Wall lies entirely in England, from Wallsend on the Tyne, now part of Newcastle, to Bowness-on-Solway, west of Carlisle. In places it runs far south of today’s Scottish border, and it never marked that border: Scotland did not yet exist. Hadrian’s successor, Antoninus Pius, moved the frontier north around AD 142 and built a turf wall across central Scotland, the Antonine Wall. It was given up within a generation, and by 164 the army was back on Hadrian’s line, which stayed the frontier, with brief interruptions, until Roman rule in Britain ended. The Wall has been a World Heritage Site since 1987.