The Battle of Mons Graupius took place in AD 83 or 84. Gnaeus Julius Agricola, the Roman governor had sent his fleet ahead to panic the Caledonians, and, with light infantry reinforced with British auxiliaries, reached the site, which he found occupied by the enemy.

Even though the Romans were outnumbered in their campaign against the tribes of Britain, they had difficulties in getting their foes to face them in open battle. The Caledonians were the last to be subdued. After many years of avoiding the fight, the Caledonians were forced to join battle when the Romans marched on the main granaries of the Caledonians, just as they had been filled from the harvest. The Caledonians had no choice but to fight, or starve over the next winter.

The Caledonian irregulars were no match for the discipline of the legions. It is estimated that a total of 20,000 Romans faced 30,000 Caledonian warriors, and a further assembly of wives and children.

The allied auxiliary infantry, 8,000 in number, were in the centre, while 3,000 cavalry were at the flanks. The Roman Legionaries were in front of their camp wall, being kept in reserve. The Caledonian army under Calgacus was stationed on higher ground; its vanguard was on the level ground, but the other ranks rose in tiers, up the slope of the hill in a horseshoe formation.

After a brief exchange of missiles, Agricola ordered auxiliaries to close with the enemy. The Caledonians were pushed back up the hill. Those at the top attempted an outflanking movement, but were themselves outflanked by Roman cavalry. The Caledonians were then comprehensively routed and fled for the shelter of nearby woodland, but were relentlessly pursued by well-organised Roman units.

It is said that the Roman Legions took no part in the battle, being held in reserve throughout. The successful auxiliaries had been recruited from the Batavii tribe. According to Tacitus, 10,000 Caledonian lives were lost at a cost of only 360 Romans. 20,000 Caledonians escaped and Roman scouts were unable to locate them the next morning.

Following this final battle, it was proclaimed that Agricola had finally subdued all the tribes of Britain. Soon after he was recalled to Rome, and his post passed to Sallustius Lucullus. It is likely that Rome intended to complete the conquest but that military requirements elsewhere in the empire necessitated a troop withdrawal and the initiative was lost. That Agricola won the battle but failed to neutralise the threat to Roman security in the north of Britain had serious consequences for the remainder of the period of occupation.

Tacitus' proud statement on the battle Perdomita Britannia et statim missa (Britain was completely conquered and immediately let go) has led to much discussion. Its implication that Agricola had defeated the last of British resistance is untrue as the uneasy peace of the next few decades proves. The suggestion that it was passed intact and peaceful to his successor is undermined by the construction and occupation of the Glen Forts and Inchtuthil in succeeding years which indicate an attempt to fence in rather than defeat the Caledonians completely.

The site of the battle is unknown but presumably lies in the Scottish Highlands, and Bennachie in Aberdeenshire on the border between the Highlands and the Lowlands has been suggested as the exact site. It has also been suggested that the decisive victory reported by Tacitus is an exaggeration, either by Tacitus himself, or by Agricola, for political reasons.