The island the Romans called Mona — Môn in modern Welsh, Anglesey in English — was not, in the year 60 CE, militarily decisive. It produced grain. It traded with Ireland. What it was, in the imagination of Roman Britain, was a sanctuary: the western refuge of the Druids, the Celtic priestly class whose authority crossed tribal lines and whose religious teaching the Romans regarded as a politically dangerous form of resistance. Suppressing the Druids was not optional for an empire that demanded religious assimilation of its subjects. The Provincial governor Suetonius Paulinus, fresh from campaigning in north Wales, set out to finish what the conquest of Britain had begun a generation earlier. He had flat-bottomed boats built for the shallows of the Strait. The infantry would cross by boat; the cavalry would ford or swim alongside their horses.

Tacitus is the only Roman source for what happened next, and he was writing decades later, with a son-in-law (Agricola) to flatter. The dramatic stage description — the women like Furies, the cursing Druids — should be read for what it is: imperial literature, designed to make the conquered look exotic and the conquerors look brave. But the violence behind the rhetoric is not in doubt. Tacitus tells us, plainly, that after the legionaries rallied they 'smote down all resistance, and wrapped the foe in the flames of his own brands.' Druids, warriors, and the women who had stood with them on the shore were killed. Then, methodically, the Romans destroyed the sacred groves. These were the religious heart of the island, places where, according to Tacitus, the Britons made offerings — including, he claims, human ones. Whatever the truth of those claims, the felling of the groves was a deliberate act of cultural extinction. A priesthood that had taken centuries to train, an oral tradition that lived only in the memory of its practitioners, was extinguished in a few days of fire and steel. No sacred groves have yet been confirmed by archaeology on Anglesey, but the late Iron Age ritual deposits found in Llyn Cerrig Bach — swords, chariot fittings, slave chains thrown into a lake as offerings — testify to the world the Romans came to destroy.

Suetonius did not finish the job. As his troops were beginning to garrison Anglesey, messengers reached him with news that would change British history: in the south, Boudica of the Iceni had risen in revolt, and Camulodunum (Colchester) was burning. Suetonius marched fast for Londinium, leaving Mona behind. For sixteen years the island lived without Rome. Then in 77 CE, Gnaeus Julius Agricola took the governorship, finished a campaign against the Ordovices on the mainland, and turned to the strait. This time there was no fleet, no time wasted. Agricola's auxiliary troops — Britons themselves, recruited from peoples who knew tidal water — swam the strait without baggage, in full surprise, while the defenders were still looking for ships. According to Tacitus, the islanders sued for peace immediately. Mona became part of the Roman province, and stayed Roman for three centuries.

Roman rule on Anglesey left a strangely light footprint. Archaeologists have found the post-invasion fortlet that overlooks Cemlyn Bay, dated by coins of Nerva and Hadrian; a civilian trading settlement at Tai Cochion on the Menai Strait, opposite the auxiliary fort of Segontium across the water; round huts that continued to be built and lived in much as they had been before the conquest. What they have not found, despite three centuries of imperial presence, is anything resembling a Roman city — no villas, no forum, no Romano-British civic culture of the kind that flourished in southern Britain. Mona produced grain and probably copper from Parys Mountain. It was administered. It was not, in any meaningful sense, romanised. By the fourth century, raids from across the Irish Sea had become enough of a threat that the Romans built a new shore-fort at Caer Gybi, on what is now Holyhead, and a watchtower may have stood at Pen Bryn-yr-Eglwys on the northwest corner of the island. Saint Patrick was reportedly taken into Irish slavery from somewhere near this coast around 400 CE.

The Romans wrote the only history of these events. The Druids' books, if they ever had any, were burned with their groves; their oral tradition died with the people who carried it. What survives is the Welsh language, still spoken by roughly fifty thousand people on Anglesey today, and the place-names that mark the landscape where the invasions happened. In the parish of Llanidan, near the south-western end of the strait, locals in 1867 still knew two field names: Maes Hir Gad, the Field of Long Battle, and Cae Oer Waedd, the Field of Bitter Lamentation. Whether these mark the actual site of a Roman attack or simply preserve the folk memory of one, the names are themselves a kind of testimony: that something happened here that the people who lived on this land afterwards did not forget.

The conflict zone is centred at approximately 53.28°N, 4.33°W — the southern Menai Strait between Anglesey and the Snowdonian mainland. The strait is roughly 8 km long, only a few hundred metres wide at its narrowest crossings, and its tidal currents can reach 7.5 knots. From altitude on a clear day the geography of the campaign reveals itself: the Lavan Sands at the northeast end of the strait where Paulinus may have crossed; the Llanidan area to the southwest where local field names hint at the battles; Segontium (Caernarfon) on the mainland side; and Cemlyn Bay on Anglesey's north coast, where the Roman fortlet still survives as cropmark. RAF Valley (ICAO EGOV) lies 25 km to the northwest. Holyhead Mountain dominates the western skyline.